ruby/rexml: REXML is an XML toolkit for Ruby

REXML was inspired by the Electric XML library for Java, which features an easy-to-use API, small size, and speed. Hopefully, REXML, designed with the same philosophy, has these same features. I've tried to keep the API as intuitive as possible, and have followed the Ruby methodology for method naming and code flow, rather than mirroring the Java API. REXML supports both tree and stream document parsing. Stream parsing is faster (about 1.5 times as fast). However, with stream parsing, you don't get access to features such as XPath.