Add #all_tags_counts method that returns distinct tag names and their usage count

[cblock]

This should return an array of arrays of the form:

[ ['tag_name_1',12], ['tag name 2', 1], [...] ]

It should work within scoped queries

Foo.all_tags_counts
Foo.where(name: 'test').all_tags_counts
Foo.all_colors_counts
Foo.where(name: 'test').all_colors_counts

...and mabye there's a better name for this method?

[cblock]

...just revised the issue description so that it better conforms to the tag naming convention.

[markbates]

I'll happily accept a pull request for this.