Fixes #99 - Adding UMD support

Hi.

I'm sending this PR as it seems like the best way to make all happy.

In resume, this PR adds support to UMD to Rickshaw. UMD, to anyone who not knows it, is the acronym to Universal Module Definition. With it, the support to CommonJS is maintaned, to global variable in browser too and...to AMD too. \o/ 

I know that maybe this is not the best way to solve the problem, but as concatenating all the files in build process (for me, using `Grunt` with `grunt-browserify` seems like the best way to improve the build process) not seems the best way, too, I thing that way may be good.

Can anyone review this? :)

Thanks!