Added a Podspec 

I added a Podspec that points to the master branch of ShareKit/ShareKit.   It works.  I have a test project that you can use to verify.

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Note: I tried pointing it at v2.0, but was running into an issue with **LegacySHKConfiguration.m**.  This file does not build because it depends on SHKConfig.h which is no longer present.  I exclude it in the current Podspec (using 'exclude_files'), and the pod installs.  However, if I include the tag v2.0 in the podspec, Cocoapods seems to ignore the exclude_files directive, and tries to compile it.  

One solution to this is to exclude the file; Is that feasible?  I think we will then be able to point at a tag in the podspec.