Allow java vm flags to be passed in

One of the smaller servers I use yui-compressor on seems to blow up unless I set the -Xmx64M flag. This is just a small commit to allow the initializer to accept <code>:java_opts</code>. So you can do something like:

<code>YUI::JavaScriptCompressor.new(:java_opts => '-Xmx64M')</code>