Allow optional fields in MsgVersion decode.

This commit modifies the MsgVersion.BtcDecode function to match the
behavior where fields after the first address field (AddrYou) are optional
and only read if the buffer contains remaining bytes.

Unfortunately this means the reader for MsgVersion.BtcDecode must be a
*bytes.Buffer or an error is returned.  This is not an issue for the vast
majority of cases since all of the message reading code which is the main
way messages are read is already using a *bytes.Buffer, however, this
change might affect external callers if they are doing something special
with custom readers.

Fixes #14.