Deprecate methods that are internally using MR_contextForCurrentThread.

I started to use MagicalRecord (2.3 beta) some days ago. I had problems related to the use of contextForCurrentThread with saveWithBlock. I understood, reading this article http://saulmora.com/2013/09/15/why-contextforcurrentthread-doesn-t-work-in-magicalrecord/, 
why contextForCurrentThread is deprecated.

The problem is that there are many methods that internally are using contextWithCurrentThread and ,especially for new users, is not easy to understand where and why they should avoid them.
I added a specific deprecate warning to those method in this pull request.

Adding the warning i also found some errors in methods where it was still used contextForCurrent Thread instead that the context provided as parameter.