Replacing &blck.call with yield

Based on performance tests from lotus/view#33, replacing block.call
with yield

I have modified the documentation to include the @yield and
@yieldparam from the Yardoc tags (see below):

http://rubydoc.info/gems/yard/file/docs/Tags.md#yield
Merge pull request #31 from jeremyf/replacing-amp-block-with-yield

Replacing &blck.call with yield