Make jsdom an optional dependency.

Other doms (notably domino) are supported now and jsdom has native
dependency that some environments fail to build.
v0.3.15
corrected repository information in package.json. It should be singular
Fix syntax error in package.json
Performance improvement of ElementStack.inScope()
Set foreign attribute prefix
Emit Characters instead of SpaceCharacters, refactor TreeBuilder

Add active formatting element limit check. Fix adoptionAgencyEndTag bug.
Order insertion modes.
Handle UTF-16 characters in EntityParser
Fix serializing for test

Use localName instead of nodeName, do not serialize attribute namespaces
Use jsdom object constructors insead of public DOM factory methods
Actually rename files (CamelCase.js)
Remove streaming-test temporary
Fix message mistypes.
Fix attribute merging in SAXTreeBuilder
Fix character handing in after[After]Body modes
Fix empty attributes (use empty array, not object)