Upgrade dependency from jwcrypto to browserid-crypto.

This is a simple search-and-replace-style update from jwcrypto v0.5.0 to browserid-crypto v0.7.0.  All the APIs are the same, but this will remove support for "new-style" certificate formats that was present in jwcrypto.  Any `BIDPublicKey` or `BIDSecretKey` values that have a `kty` field will not be compatible with this change, and should be re-formatted into the "old-style" data format.