Implement data persistence
Add update() method
Add NotModifiedException class
Handle response code 304
Implement conditional request
Fix update
Add test record helper
Add test case for conditional request
Merge branch 'develop' into topic/ConditionalRequest

Conflicts:
	.gitignore
	github/Requester.py
Review of #189: use dict.get

http://docs.python.org/2/library/stdtypes.html#dict.get
Review of #189: pep8, copyrights, style, remarks

For remarks, run: git grep "#189"
They are only my first thoughts while reviewing this pull request,
and should be reviewed themselves.
Fix remarks on #189 to #193
Remove _record_.py (#193)

According to https://github.com/jacquev6/PyGithub/commit/0f74e4389b3c0fa57a83083ecfbbf5c331022674#commitcomment-3919786
Separate tests for conditional requests (#193)
#193: Don't use a try-except for a usual execution flow in GithubObject.update

(and factorize assignment of headers in _storeAndUseAttributes,
as done for rawData)
#193: Add remarks
#193: Add a param to Requester.requestXxx for request headers
#193: Fix test coverage
Move method update to CompletableGithubObject
Move the DEBUG_ON_RESPONSE call to Requester.__requestEncode