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.