get/set with [] and []=

Use ArgumentError for missing attributes
Remove explicit return statements
Have #set return the value that is set, not the int
Allow setting the bitmask attributes via a Hash
remove require from gemspec

I don't think it is needed and rake can conflict with other gems
sometimes (such as capistrano)
remove dependency on Rake in gemspec
Merge pull request #1 from polleverywhere/master

Various suggested tweaks