Automatically create the download-cache directory if missing

This fixes the issue #171.

src/zc/buildout/buildout.py

@@ -351,12 +351,8 @@ class Buildout(DictMixin):
         download_cache = options.get('download-cache')
         if download_cache:
             download_cache = os.path.join(options['directory'], download_cache)
-            if not os.path.isdir(download_cache):
-                raise zc.buildout.UserError(
-                    'The specified download cache:\n'
-                    '%r\n'
-                    "Doesn't exist.\n"
-                    % download_cache)
+            if not os.path.exists(download_cache):
+                os.mkdir(download_cache)
             download_cache = os.path.join(download_cache, 'dist')
             if not os.path.isdir(download_cache):
                 os.mkdir(download_cache)

src/zc/buildout/downloadcache.txt

@@ -139,3 +139,23 @@ install-from-cache option set to true:
     Getting distribution for 'demoneeded'.
     Got demoneeded 1.1.
     Generated script '/sample-buildout/bin/demo'.
+
+Auto-creation of download cache directory
+-----------------------------------------
+
+With zc.buildout version 2.2.2 or higher the cache directory is automatically
+created, provided it is within an already existing directory::
+
+    >>> write('buildout.cfg',
+    ... '''
+    ... [buildout]
+    ... parts =
+    ... download-cache = %(cache)s/newdir
+    ... ''' % globals())
+
+    >>> print_(system(buildout), end='')
+    Uninstalling eggs.
+
+    >>> ls(cache)
+    d  dist
+    d  newdir