Don't convert user supplied include to symbols

Avoids a possible memory leak by leaving the user specified params[:include]
as a string. Instead adds a `::can_include?` method that tests specified
includes as both strings and symbols.

Closes #85
Allow customization of has_many link parameters

Can pass hash to can_include values with param and value keys that
override the defaults of `model_id` and `{model.id}` in to-many links.
Merge branch 'fix-memory-leak'