Implement :all_on_start option, plus some other things

Similar to guard-rspec, I wanted the ability for jslint to run right when I fire up guard, so this branch has an implementation of that, including the add to the README.

Also, the "require "guard/jslint-on-rails"" was problematic for ruby > 1.9.2:

```
There was a LoadError while loading guard-jslint-on-rails.gemspec:
cannot load such file -- guard from
  /Users/jhagglund/.rvm/gems/ruby-1.9.3-p547@citysearch/bundler/gems/guard-jslint-on-rails-ba1af46bed9f/guard-jslint-on-rails.gemspec:3:in `<main>'

Does it try to require a relative path? That's been removed in Ruby 1.9.
```

So my solution to that was to just put the version directly in the gemspec, which solved the issue.

Lastly, replaced .rvmrc with .ruby-gemset and .ruby-version.

Thanks a ton for writing this, by the way! Very happy to have jslint ability in our javascript-heavy rails apps.