For Wendelin project I needed to rework BigFile interface to support on-server data appension. But while doing so I've discovered in many places current BigFile code could crash and is not working properly. Thus this patch series contain:

1) fixes for discovered bugs;
2) support for on-server data append via exposing ._dataAppend(data_chunk) method;
3) basic tests for BigFile (so that we know fixes are proper and we won't regress on
   the same things in the future).

Newly introduced BigFile tests were verified to pass on the testrunner:

https://nexedi.erp5.net/test_result_module/20150303-A925D6B https://nexedi.erp5.net/test_result_module/20150303-A925D6B/7

NOTE running the same testsuite previously also passed for BigFile tests:

https://nexedi.erp5.net/test_result_module/20150303-13B4A549 https://nexedi.erp5.net/test_result_module/20150303-13B4A549/8

but whole testsuite failed because of:

1. erp5_test_result:testTaskDistribution failure https://nexedi.erp5.net/test_result_module/20150303-13B4A549/24 , and

2. testERP5BankingCounterRendering failure https://nexedi.erp5.net/test_result_module/20150303-13B4A549/110

testTaskDistribution is currently failing from time-to-rime even on ERP5-MASTER tests, e.g. here https://nexedi.erp5.net/test_result_module/20150226-1F4459D0, and testERPBankingCounterRendering test passed on the second whole test run and passes on my personal slaprunner, so maybe that test or code it tests is also flaky, and from my point of view that should not be related to BigFile at all.

Please pull.

Thanks beforehand, Kirill