Simplify debugger classes....
Simplify debugger classes.
Directly get the color scheme from the interactive shell (which is
created if not already present, anyways). The goal is ultimately to
allow the IPython debugger to be a drop-in replacement of the stdlib pdb
with the same signature (in particular, this would obviate the need for
plugins such as nose-ipdb and pytest-ipdb; nose and pytest could instead
let their `--pdb` command line arg take an argument specifying the Pdb
(sub)class to use, e.g. `py.test --pdb=IPython.core.debugger.Pdb`).
Also deprecate the `Tracer` class (and related utilities), which can be
fairly trivially replaced by `IPython.core.debugger.Pdb().set_trace()`
(we can let `Pdb` catch `BdbQuit` itself).
