Set version to 0.26.0
HTTP support improvements:
* RFC2616 compliance
* Case insensitive equality, inequality operators for strings
* Improvements to message_parser
* Tidy up HTTP method enumeration
* boost::asio buffer helpers
HTTP handshake in peer protocol (RIPD-351):
* New I/O paths for client and server role
* New handshake_analyzer detects the peer protocol
* New basic_message class for parsing and storing HTTP messages
* Conditional compilation for selective feature enabling.