Configure absolute and server URL prefixes

[manics]

This started off as a fix for the X-Forwarded-Context X-ProxyContextPath. Previously they were always /proxy/{port}, now they should be /server-name if this is a managed server. Since this is related to the code that controls the URL rewriting I thought I'd add in support for absolute URLs.

I've done some testing with:

c.ServerProxy.servers = {
    'python-http': {
        'command': ['python3', '-m', 'http.server', '{port}'],
    },
    'python-http-abs': {
        'command': ['python3', '-m', 'http.server', '{port}'],
        'rewrite': '',
    },
}

python-http should produce logs like

"GET / HTTP/1.1" 200 -

python-http-abs should produce logs like

"GET /python-http-abs/ HTTP/1.1" 404 -

Also /proxy-abs/port is the absolute version of /proxy/port

Needs more testing! @jacobtomlinson @psychemedia

Closes #43

[manics]

Last two commits add the changes suggested by @yuvipanda (#84 (comment)) and adds a travis test

[yuvipanda]

@manics I've made the following changes here:

1. Call it absolute_url and make it boolean, instead of rewrite
2. Make it work with websockets too

This is an amazing contribution, thank you very much for it!

I'll merge shortly

[yuvipanda]

@manics I'm actually going to wait for you to OK my changes before merging.

[manics]

@yuvipanda I'm OK with your changes, just need to fix Travis.

[yuvipanda]

Am also super thankful for adding tests :) Would be nice to switch to Circle at some point though.

[yuvipanda]

@manics I've made a new release (1.0.0b9) with your changes! \o/

Should really make a 1.0 release #86