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
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