Allow dynamic keys via as: proc

[james2m]

This adds the ability to specify keys dynamically in the expose as: using a proc. This uses the syntax as preferred by @dblock in #172

  object = OpenStruct(awesomness: 'awesome_key', awesome: 'not-my-key' )

  class MyEntity < Grape::Entity
    expose :awesome, as: -> { object.awesomness }
  end

  => { 'awesome_key': 'not-my-key' }

I've included #264 as it made seeing the wood for the trees easier when testing.

[LeFnord]

thanks @james2m … I prefer to first finish #264 and mörge it, then this one, ok?

[james2m]

@LeFnord Of course.

[james2m]

@LeFnord I'll rebase this when #264 is merged so you can see the wood for the trees.

[coveralls]

Coverage decreased (-0.07%) to 92.72% when pulling 68baab0 on james2m:dynamic-keys into 0930f60 on ruby-grape:master.

[james2m]

@LeFnord done, I'll stop fiddling now.