Add find_by_id method to ModelAdditions

See https://github.com/tute/merit/issues/13
Merge pull request #14 from potomak/master

`find_by_id` method missing error with Mongoid
Releasing 0.8.1
Ruby 1.8.7 Proc#arity fix.
Better points granting doc.
Adding allow_multiple boolean option to Badge#grant_to
Releasing 0.9.0
Inferring params[:id] from target_object.id when not present
Releasing 0.9.1
bug fix
Releasing 0.9.2
Move the generation of the core tables to install

I wanted to merit multiple models. I'm not sure if this is going to
work in the long run, but the generators wouldn't let me run:

    rails g merit user
    rails g merit post

The merit generator was also creating the migrations for the core
tables, like badges and sashes. I moved that to the install
generator and left only the model specific stuff in merit.
add an example model name for in the readme for the generator
user_model_name can be customized in config
don't throw exception if target_object cannot be retrieved in merit_action - this object is not needed if no block or no-parameters block is given
README update (grant_to :allow_multiple option). Default orm option for install generators.
Remove Abstract User from To-do list
Merge pull request #23 from tomgi/customizable_action_user_class_name

Customizable action user class name
Little README update.
Its now possible to have multiple point rules for one action.