Added 3 new options to support the customizations I needed but were missing from the original plugin

Added 3 new options:

1) units - this is an array of the units to display. If the greater units are not in the displayable list the plugin will calculate their values and add it to the largest unit. e.g. if :units => [:days, :hours, :minutes] and :format => :long and the duration is "2 months 1 day and 10 hours", the output will be "61 days 10 hours".

2) keep_zero - this already existed but only for the :chrono format. I simply moved it out as an option so all the other formats can benefit from this. Was something I needed too!

3) verbose - I needed some way of distinguishing the units in the output string to give certain units more importance. The verbose option wraps spans with meta classes around the returned string value. I left the html_safe out of the plugin as that is more a task for the view.