Explicitly support Windows and test cleanup

This fixes #3 by searching for 'vagrant-wrapper' which detects both the rubygems wrapper and itself.

This fixes #2 by handling a PATH with a semi-colon as a separator, as windows does. It also adds the default path of Vagrant on windows to default_paths. 

Finally, the tests are cleaned up a bit, updated for Rspec3, and include new tests for the new methods. The tests now all pass on my Ubuntu workstation. More work would be required to get them passing on Windows.
same as above, but a bit more readable and more briefly:
```ruby
def is_wrapper?(file)
  File.readlines(file).grep(/#{WRAPPER_MARK}/).any?
end
```