Minor refactoring and docs

There were some small things i wanted to do for EasyMapping amongst large things like CoreData import, so i'm collecting them into this PR. Things i've done:

* Refactored EKFieldMapping dateFormat property to be a simple block, that converts to NSDate and reverse.
* Added MIT license to all EasyMapping source files
* Added documentation to all EasyMapping classes.

This should get EasyMapping a better look on CocoaDocs web-page, where it currently does look kinda empty =) And also it allows for neat XCode and AppCode code completion. For example, here's how EKManagedObjectMapping initializers look in XCode with these changes:
http://d.pr/i/zNjd

I believe after this we should do at least minor version increase, following [Semantic Versioning](http://semver.org), since there were some breaking changes in the last release.