Moved all errors to cql/protocol.rb

and made them subclasses of ProtocolError
Moved varint (and UUID) decoding to Decoding
Removed pending tests for read_option_list!

They aren't used anywhere in the spec…
#read_varint! defaults to the buffer length as length
Moved decimal decoding to Decoding#read_decimal!
Moved long decoding to Decoding#read_long!
Removed pending specs and method stubs in Encoding

They aren't going to get implemented anytime soon since they would only be needed for a server implementation
Moved encoding of BIGINT, VARINT, DECIMAL and UUID to Encoding