Signer and Base64URL decoding fixes

1. Includes fix for Base64URL decoding padding #9 (thanks @alexaubry!)
2. Key based Signers can now be initialized with unencoded keys:
   public init(key: String)
   The existing String based initializer was modified to highlight the fact that the key is expected to be encoded:
   public init(encodedKey key: String, encoding: Encoding = Base64Encoding()) throws