Don't depend on YUI – but still support it!

CSS/JS compression is disabled by default. Issue #13 complains that the version that the gemspec requires causes issues on windows and that it is three years old.

I'm working on a project that would like to use the HTML compressor, but has no use for the CSS/JS compression. Pulling in YUI makes just the project fatter.

I moved the YUI code into separate requires. If a user wants it, they just need to include the `gem 'yui-compressor'` and `require 'htmlcompressor/yui'` -- that will recreate the behavior of 0.0.7.

I also added requires for closure (based on the code in the test), and for just the css or js part of YUI.

If a user wanted, they could use YUI for CSS and Closure for JS:

```ruby
gem 'htmlcompressor'
gem 'yui-compressor'
gem 'closure-compiler'
...
require 'htmlcompressor/yui/css'
require 'htmlcompressor/closure'
```