Compound filters

Allows for compound filter fields in the form of:

``` ruby
<%= table_for @products do |t|
  t.column [:title, :description], :filter => :like, :compound_with => :or
  t.association :vendor, [:name, :description], :filter => :like, :compound_with => :and
end %>
```

Currently only joins with `OR` sql. Might be helpful to add a `compound_with` setting that would allow us to use `OR` or `AND`.

There may be weird edge cases to this addition. Not sure what not adding the like filter will do to compound conditions.

Did not add functionality for Mongoid. Would appreciate someone who knows mongo better to jump on this. Feel free to add before pulling. Opening more for discussion than anything.