Markup.ml 0.7.1

• New trees function for converting a signal stream to a stream of
  tree data structures.
• Fixed bug in existing tree function that caused it to consume all
  signals, instead of only those needed to construct one tree.
• Comments, processing instructions, XML declarations, and doctype
  declarations can now be optionally included in tree assembly.
• Added tree deconstruction helper from_tree. This converts a tree to a
  signal stream (prompted by Angelos Oikonomopoulos).
• Added predefined namespace URIs, e.g. Markup.Ns.html is the string
  "http://www.w3.org/1999/xhtml".
• Added transform function, which maintains an accumulator like fold
  does, but produces a derived stream instead of the final value of the
  accumulator (prompted by Angelos Oikonomopoulos).
• Renamed the internal module Xml_parser, to avoid conflict with
  Xml-Light's internal module of the same name (reported by Angelos
  Oikonomopoulos).