Allow Self Love with Thumbs Up

Where is the love?

I have Users that need to vote on other Users (hypothetically, of course).

````
class User
  acts_as_voter     # People vote ...
  acts_as_voteable  # On other people
end
````

This adds two has_many relationships to the class with the second obscuring the first:

````
  has_many :votes, :as => :voter, :dependent => :destroy
  has_many :votes, :as => :voteable, :dependent => :destroy
````

With this pull request the relationship name is configurable, though by default stays as `:votes` for both, so it is fully backward compatible.  Now you can do the following in an initializer:

````
ThumbsUp.configure do |config|
  config.voteable_relationship_name = :votes_by
  config.voter_relationship_name    = :votes_on
end
````

Now on any given user:
````
u = User.first
u.votes_on # => the votes that have been cast on this user
u.votes_by # => the votes that have been cast by this user
````

Thanks for the great voting tool!

# Backwards Compatability

* 100%
* Didn't even need to alter any test code
* Defaults preserve the old behavior

# Tests

* They all pass
* I updated the README instructions with what I had to do to get it running on latest postgres, and latest mysql