Use correct shell and environment

This fixes issues ranging from RVM to fish.

Closes #3.  Closes #9.
Set xterm title with screen and tmux
Fix formatting
Fix executable tab complete on Windows
Support stable release of screen

Closes #25.
Ignore non-strings in b:dispatch

In the future, function references may also be supported.
Default :Start to b:start

I would like to provide an interface for setting the title as well, but
I'm not sure what that should look like.
Detect make programs containing hyphens

Closes #30.
Fix capture of xterm title in tmux

This technique for setting the title in xterm sometimes gets captured as
output, so abandon it for now.