Make zksigma ready for use by zkledger (apl)

This pull request superseeds #4 and contains all necessary changes for zksigma to be used by APL (zkledger)

Internalizes the special version of btcec from @narula/btcd/btcec
Fixes various typos and comments
Removes a few unused functions
Makes functions private that aren't used from outside of zksigma
Removes BulletProofs since they're not working
Fixes the problem in Rangeproofs (https://github.com/mit-dci/zksigma/commit/d4c6dd6a4e5f8d37bd95ff2ad99fef3e2f66b240)
Adds KeyGen (https://github.com/mit-dci/zksigma/commit/e1b77e73cc5c04d8a1ffb26e5ea5fd57a6d2b322)
Improved consistent interface for all proofs (create them with New....Proof() and verify with proof.Verify() )

mit-dci / zksigma