added pretty-print capabilities

Added the ability to output a formatted XML document tree, with
indentation and newlines, when outputting children and siblings,
respectively.

As is the case with all 'pretty-print' capabilities of other libraries,
the document tree output is not the same. The resultant text if parsed
will have a tree that will contain more/less XmlText nodes than the
original tree.
Merge branch 'master' of https://github.com/dirvens/dart-xml

Conflicts:
	lib/xml/nodes/branch.dart
	lib/xml/nodes/cdata.dart
	lib/xml/nodes/comment.dart
	lib/xml/nodes/doctype.dart
	lib/xml/nodes/element.dart
	lib/xml/nodes/processing.dart
	lib/xml/utils/writable.dart