Specify runtime dependencies, update README with OS X installation

Rumtime dependencies were specified as development dependencies, leading to a long installation process that could confuse folks.

To give an example, this was my installation workflow before:

```sh
2.1.1 :001 > require 'masque'
LoadError: cannot load such file -- capybara
2.1.1 :001 > quit

$ gem install capybara
$ irb
2.1.1 :001 > require 'masque'
LoadError: cannot load such file -- capybara/webkit
2.1.1 :002 > quit

$ gem install capybara-webkit
# failure
$ brew install qt
# success
$ irb
2.1.1 :001 > require 'masque'
LoadError: cannot load such file -- capybara/poltergeist
#... repeat for poltergeist and headless
```

Hopefully this PR clears it up!