Allows for request custom headers when fetching messages

* Defines new request kind ExtraHeaders
* Exposes extraHeaders property on fetch messages operations
* Modifies MessageHeader::importIMFFields for handling any headers
Fix for incorrect date() check
Fixes for PR feedback

* created separated fetchWithExtraHeaders methods, restored previous API
* removed log
* adding comments
Merge pull request #204 from thusfresh/CustomReceiveHeaders

Allows for requesting optional headers when fetching messages.
Adds automatically fetch kind when extra headers is non-NULL
Fixed xcodeproj: don't include sources of fraemwork in tests.
Remove case of Comment and Keyword headers.
Travis configuration
Fixed target names
Debug info for Travis builds
Travis: try to use xcodebuild
Try to disable code signing
Disabled links test for iphoneos platform
Fixed indentation of MCIMAPAsyncSession.h
Fixed namespace constants
Added an Obj-C method to create a file attachment with in-memory data.
insert a build status image in the readme
Fixed #227: SMTP OAuth2 can now be used even if the password is not set
Removed unused ivar
Fixed IndexSet::count() implementation.