fixes #75: Speed up tree traversal over leaf nodes

For some (unknown) reason, when trying to find a node by name/regex/substring in a Tree
or Tree subclass, the following-sibling::* XPath was being really slow (exactly 10 seconds
every time a leaf node was being examined in the "while queue" loop). I moved the XPath
following-sibling behavior out of the browser and into python; it seems much faster now.

While I was in there, I hit Tree.find_node_by_regexp with the DRY stick by seeding the queue with self instead of self.children.
Merge pull request #76 from seandst/speed-up-tree-leaf-nodes

fixes #75: Speed up tree traversal over leaf nodes