Convert vagrant-hosts to use guest capabilities

This is in preparation for adding Windows support.
Add Windows support

Allow vagrant-hosts to operate on Windows systems
Convert capabilities call to vagrant convention

Rather than returning an object on which to operate, calling the
capability executes the action, whatever it is. This modification is
being performed in order to better line up with Vagrant conventions.
Add/move i18n definition

In a previous commit an error message was changed to call a different
i18n key, but the key was not moved. This commit rectifies that change
omission.

This commit also removes the unused SyncHostsFailed error class.
Merge pull request #15 from reidmv/convert-to-capabilities

Convert vagrant-hosts to use guest capabilities