Serializer toString is now serializeToString

Serializer:
* moved the public toString method to be named serializeToString
- removed default format 'html5', as this ought to be taken care of by
the doctype (can still be overridden)
* detection of the format is now taken care of by the docType and
processingInstruction elements, no longer the documentFragment
Moved optimizations to the Serializer

Template
* removed obsolete code from the internal clean function
* added an additional 'options' argument to both the internal and
public 'render' functions
* when replacing placeholder the replacement nodes are properly
imported into the document (this is actually a DOM requirement, not a
xmldom-module requirement)