Methods to provide random and weighted random ActiveRecord::Relation scopes

[paulspringett]

Add order_by_rand and order_by_rand_weighted methods that just append an order clause without applying limits or converting to an Array or single object.

Supersedes #24 and fixes #23

[paulspringett]

Provides methods as suggested by @spilliton in #24

Artist.order_by_rand.first 
Artist.order_by_rand.limit(50).to_a
Artist.order_by_rand_weighted(:score).first
Artist.limit(5).order_by_rand_weighted(:score).to_a

[paulspringett]

@spilliton I've added the methods as you suggested and updated the existing random and random_weighted methods to make use of them (they still have the same inputs and outputs though).

[spilliton]

Looks pretty good to me, nice work! I'll fix the build, looks like it's the pesky posgres specific issue

[paulspringett]

@spilliton nice one, thanks for the merge

[spilliton]

No problem, I can hopefully get around to a release in the next couple days. I likely won't go 1.0 yet but maybe 0.5.0 or something