Add .gitignore based on .hgignore. Also ignore 'venv'.
Fixes this unit test failure under Windows:

```
======================================================================
FAIL: test_egg (tests.test_codeunit.CodeUnitTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "D:\PycharmWorkspace\coveragepy\tests\test_codeunit.py", line 102, in test_egg
    "# My egg file!"
AssertionError: '# My egg file!\r' != '# My egg file!'
```
Starting prototype of collection of test callers for each statement.  The data is collected and printed to stdout during collection, but isn't currently saved to disk or used anywhere.