purescript-signal now pulled from bower.

Removed my copy of the purescript-signal code and started using the copy
on bower.  It wasn't up on bower when I initially started, so had to
copy the code into my project.
Cleaned up the code a bit.

- Removed some redundant code
- Removed purescript-node-thunk dependency.
- Moved stanza parsing code into Xmpp.purs