Jedi debug facility.

The Jedi library has its own debug tracing facility. We're now
using this to provide further information about bugs in the library,
to make it easier for the Jedi authors to reproduce them.

Implements #219.
The new navigation only work in Emacs 24.3 and newer.

The old Python extension in 24.2 and earlier lacks the required
navigation functions, and backporting them would take too much
code.