Group public headers into Public directories

[paulb777]

Make public header organization in podspec's more robust and manageable. Issue #23.

[XiangtianDai]

Do you plan to have a follow-up to get rid of the "Private/" subdirectory?

[paulb777]

@XiangtianDai I could follow up and contents of the Private subdirectory to their previous locations. Do you want that?

[XiangtianDai]

@paulb777 What I wish is to get rid of the apparent inconsistency between top-level files under Source/ and files under subdirectories. Right now (in your branch) all top-level .h files are in either Public/ or Private/ while .m files sit directly in Source/, however in a subdirectory, say, Source/RPCs, .m files are private .h files are mixed together. Not a big deal, but it would be nice to have a cleaner way to structure them.

[paulb777]

@XiangtianDai The only requirement from a unit testing perspective is that public headers are in separate directories from private headers, so that module headers don't collide with the source versions. I'll do a follow up PR to restore the headers from Private and leave any additional reorganization to you.