Don't depend on YUI, instead offer requires that configure YUI or Closure as the default CSS/JS compressor
Update README
add a MissingCompressorError exception
Update closure test for `closure-compiler 1.1.10` (, -> ;)
Automatic require for yui and closure compilers

- no need to explicitly require htmlcompressor/yui or
  htmlcompressor/closure
- symbol shortcut for yui and closure compilers in javascript_compressor
  and css_compressor options
Update README to document changes in options
remove unnecesary reset_default_options!