LRDOCS-526 PropertiesDocIndexBuilder

@jrao

Hi Hugo,
After we use the PropertiesDocBuilder to build properties HTML files to a directory, we use this class to build an HTML file (index.xml) as an index to those properties files.

The index.html file gets written to the same directory as the properties files it references. That way, eventually, we'll be able to upload the whole directory to docs.liferay.com.

We have changes to portal-impl/build.xml that we'll send after these classes get into trunk.

Thanks,
Jim