Implemented find_related_tags(:ignore => ['foo', 'bar'])

Hi,
At times I need to explicitly exclude a tag from the related_tags search to improve the result-quality (think of tags that are very common but skew the results in unforseen ways).

So I decided to implement this as a special key in the options of `related_tags_for called` `:ignore`.
I included some tests, please let me know if I did something insanely stupid :)

Works with single values, symbols and arrays of values/symbols.