test file in test_args
responses.add_callback method
test add_callback
updated README
Merge pull request #24 from pmdz/add_callback

Response callback - body as callable
Add support for regex URL matching
Fix linter errors
Improve regex support retaining backwards compat.
Fix Py2-Py3 compat. Add another test case.
Remove the unicode test. Sigh..
Add another test and fix a missing match case
Add documentation about regex matching
Merge pull request #25 from karls/regex-url-matching

Add support for regex URL matching
Merged dropbox:master into ryaanwells:master
Updated error mocking to overload body variable
Readme has description of error mocking.