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).