Drop Ruby 1.8 support

Capybara 2.0 dropped 1.8 support, so Domino probably should also.
Merge pull request #7 from ersatzryan/drop-1-8-support

Drop Ruby 1.8 support
Add ability to pass block to attribute

This allows for the ability to modify/typecast the string coming back
from Capybara.
Merge pull request #8 from ersatzryan/callbacks

Add ability to pass block to attribute
Version 0.4.0