Add JRuby's iconv ext.

This PR pulls in the iconv impl from JRuby 1.7 and adds appropriate build and gemspec bits to build a -java version of the iconv gem.

We would like for users still dependent on iconv to be able to install the iconv gem in the future. JRuby 9000, which will be Ruby 2.2 compatible, will not ship iconv.

The JRuby version of iconv uses Java's charset subsystem in this version. That means all transcoding passes through UTF-16. We may want to do a version that uses the Ruby transcoder subsystem (Encoding::Converter and related code).

Tests run but there are three failures. We can address those outside this PR, since all known users of JRuby's iconv are happy with the current level of compatibility.