Doesn't yet implement the rules in the docs, but I think the doc rules
could maybe change to match this. Needs lots of cleanup work and
thorough testing, but this is a great start.

* Don't implement the rules for when preloading occurs yet
* Don't attempt to streamily preload modules downstream while this
  context hasn't yet received the final module. There is quite
  significant latency buried in here, but for now it's a lot of work to
  fix.

This works well enough to handle at least the mitogen package, but it's
likely broken for anything bigger.

For gevent, just as in 5f7633cd569f671750ea7f35f6ca79118b345205

It seems gevent automatically sets blocking behaviour on fds produced by
the socket module, which causes the Python process we fork to fail
horribly. So in the child, always reset the blocking flag.

..and enable privileged=True since that's needed for ptrace()

Closes #70

Either execvp succeeds, in which case the process image is replaced, or
execvp throws an exception, in which case control flow exits the frame
anyway.

e.g. assert x == y -> self.assertEqual(x, y);
self.assertTrue(isinstance(x, y)) -> self.assertIsInstance(x, y)

These specific methods give more useful errors in the case of a test
failure.

This allows using features such as TestCase,assertIsInstance on Python
2.6 and earlier.

Although these are synonyms in Python 2.x, when using MyPy to typecheck
code use of file() causes spurious errors.

This commit also serves as one small step to Python 3.x compatibility,
since 3.x removes the file() builtin.

We use test.sh to consolidate test cases in one file so the README
would not be outdated easier rather than the hard-coded test commands
in a code block.

On my laptop (Ubuntu 17.10, Python 2.7.14 in a virtualenv),
`test_regular_mod` fails with

```
AssertionError: "\nimport sys\n\n\ndef say_hi():\n    print 'hi'\n" !=
'\x03\xf3\r\n\xbbW\xd5Yc\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\x00\x00@\x00\x00\x00s\x19\x00\x00\x00d\x00\x00d\x01\x00l\x00\x00Z\x00\x00d\x02\x00\x84\x00\x00Z\x01\x00d\x01\x00S(\x03\x00\x00\x00i\xff\xff\xff\xffNc\x00\x00\x00\x00\x00\x00\x00\x00\x01\x00\x00\x00C\x00\x00\x00s\t\x00\x00\x00d\x01\x00GHd\x00\x00S(\x02\x00\x00\x00Nt\x02\x00\x00\x00hi(\x00\x00\x00\x00(\x00\x00\x00\x00(\x00\x00\x00\x00(\x00\x00\x00\x00sF\x00\x00\x00/home/alex/src/mitogen/tests/data/module_finder_testmod/regular_mod.pyt\x06\x00\x00\x00say_hi\x05\x00\x00\x00s\x02\x00\x00\x00\x00\x01(\x02\x00\x00\x00t\x03\x00\x00\x00sysR\x01\x00\x00\x00(\x00\x00\x00\x00(\x00\x00\x00\x00(\x00\x00\x00\x00sF\x00\x00\x00/home/alex/src/mitogen/tests/data/module_finder_testmod/regular_mod.pyt\x08\x00\x00\x00<module>\x02\x00\x00\x00s\x02\x00\x00\x00\x0c\x03'
```

`__file__` contains the path of the compiled `.pyc`, not the `.py`
source file.

fixes #58

Since the above if block ends in a call to os.execv() this block will
only ever run when the if condition was false. Hence putting it in an
else clause is unnecessary.