0.1.0

• #16: Added ability to override elementMap, namespaceMap and baseUri for
  a fragment of a document during reading an writing using pushContext and
  popContext.
• Removed: Writer::$context and Reader::$context.
• #15: Added Reader::$baseUri to match Writer::$baseUri.
• #20: Allow callbacks to be used instead of Element classes in the Reader.
• #25: Added readText to quickly grab all text from a node and advance the
  reader to the next node.
• #15: Added Sabre\XML\Element\Uri.