Add Zeus support, smartly decide to use bundler or zeus

Hi guys,

This pull request is very similar to #28, I've been using it since last week and I haven't experienced any problems with it.

The main difference between this and #28, is that we detect if zeus is running by the presence of `.zeus.sock` in the project folder.

There one new setting `g:vroom_use_zeus`, if it's equal to 0, Zeus will not be used at all regardless of the presence of `.zeus.sock`