Remove dead code and start adding documentation comments
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
Add support for missing XML node types

Adds the missing node types needed for outputting complete XML documents
(namely, comment and processing-instruction nodes).

All seven node types defined in the XML spec can now be read, created,
and serialized using gokogiri.
Run go fmt
Documentation for  NodeById and add a test
Merge pull request #48 from jbowtie/missing-node-types

Missing node types (comment and processing-instruction)