Enable and document control over serialization

Add Node.SerializeWithFormat to enable control over formatting.
Add Node.ToUnformattedXml as a convenience for serializing an
unformatted string.
Document Node.ToXml and Node.ToHtml
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)
adding NodeById html test
Tests for SerializeWithFormat, ToUnformattedXml
Change type of format argument to SerializationOption
Merge pull request #43 from jbowtie/serialization

Add control over serialization format issue #40
Updating interfaces
fixing compile time error on test
fixing tests, unformatted does not mean strips whitespaces, it means the format does not change.