Fix prefix regexp

First of all, thanks for sockjs-go!
I have started using it and it looks pretty good!

I was taking a look at the way prefix is handled internally and thought:

- prefixes are interpreted as regexps, but it is not obvious to the user: they should be escaped
- force the prefix regexp to start with ```^``` so that ```/prefix/``` matches but ```/not_prefix/prefix/``` doesn't.
- remove trailing ```/``` from prefix by using path.Clean so ```prefix+"/something"``` avoids double slashes.
- removed (apparently) unecessary ```?``` from ```/info?$``` regexps
- appended ```$``` to some regexps (apparently) missing it

Please let me know if I incorrectly interpreted any of the points above.