Add support for passing mount options to the Mount resource.

Sometimes the cookbook implementer needs to pass comma-delimited options to the Mount command, for example, 'nfsvers=3'.
Remove String interpolation from mount options. Remove the `join(',')` too. The `options` declaration supports both Array and Strings. Update README accordingly.
Merge pull request #27 from rbrooks/master

Add support for passing mount options to the Mount resource.