Update README.md

Improved read me page for newcomers so that they easily find out, what is share kit about
BUGFIX: Facebook iOS sharer presents alert, when there are not accounts in settings.app
DOCUMENTATION: sharer matrix updated
Merge branch 'master' of github.com:ShareKit/ShareKit
all targets now compile for 64bit too, deployment target raised to 5.1.1. The only exception is Buffer, which framework is not yet compiled for 64bit.

SUBMUDULE UPDATE: Buffer, Evernote, google api, google toolbox
WARNING FIXES: for hassle free 64bit compilation
removed buffer sharer temporarily, until they release 64bit library.