Fixed podspec, added subspec for the receipt validator and removed OpenSSL source.

The podspec didn't work so I fixed it, I created a subspec for the receipt validator which depends on OpenSLL, so you don't have to build OpenSSL if you just want to use the SCPStoreKitManager. I also removed all source for OpenSSL for security reasons.

I did not update the example nor the readme.

Nice work btw :)