Add symbol conditionals

Now you can give symbols as conditions for :if and :unless, which are
used as method names for the object in question.

Example:

   collection @posts
   attribute :coolness, if: :cool?

Here Post#cool? would be invoked when resolving the condition, to
determine whether to include the :coolness attribute or not.
Refactor the conditional statement

Now call_condition_proc does the checks whether a proc is to be called.
The block that may be given projects the returns value. By default it is
the identity, but for `:unless` we need to negate the results.
Update README.md
Merge pull request #453 from leoc/master

Add symbol conditions