the new function AddUserAgent adds the user agent to the stack
and formats it as per BIP 0014
e.g: "/btcwire:0.1.4/myclient:1.2.3(optional; comments)/"
the validation on UserAgent has been moved to a new function
validateUserAgent
There was not much documentation about the difference between testnet and
testnet3, so make it clear that testnet is used for regression tests and
testnet3 is the public test network (version 3).
This commit adds a new MessageError type that is intended to allow the
caller to differentiate between general io errors and and errors that
resulted from malformed messages.
