Expose base.py as a public api.

[arenevier]

Extract methods that can be used to implement Websocket over HTTP/2

The implementation of HTTP/2 handshake is quite similar to HTTP/1 with a few differences:

• headers are lower case in HTTP/2
• not exactly the same headers must be validated
• We don't handle Sec-WebSocket-Accept for HTTP/2
• the response is different

So we can implement the handshake in a base class that will delegate some operations to the parent class.

We make _base.py public by renaming it to base.py

[arenevier]

two notes:

• I decide to rename _base.py to base.py because: a) it now contains a base class b) there is already an util.py in mod_pywebsocket/ so I figured it would be confusing
• Here is what HTTP/2 handshaker would look like in wpt with that change: https://gist.github.com/arenevier/1845daecc3dccad278b17e7015f5aa8f#file-ws_h2_handshake-py