Errors in compressor not raised on the Ruby side.

 If the Java yuicompressor reported an error, it would not raise an exception on the Ruby side. This is because the Open3.popen3 doesn't actually raise an exception in the block. Also added test coverage for the error case.

Fixed this with using Open3.capture3 instead which reports job status explicitly.
Open3.capture3 takes a string as input and returns a string. Added `stringify` method to convert strings.

(Side Note: If using streams, all streams should probably be opened in "binary" mode explicitly to avoid
further UTF-8 gliches. This might still be a problem in the jRuby code path.)

Bumped version to 1.3.3