This reverts commit af902b50.

As I was told: Please remember that 'bin/cython' is the script that
gets installed in '/usr/bin'. Why should '/usr' git injected in
sys.path?

Made tests less verbose by not using distutils.core.setup + skip debugger tests when gdb < 7.2 or not available

Made the distutils extension 2.3 compatible
Renamed the --debug flag to --gdb and --pyrex-debug to --pyrex-gdb

python code stepping (for libpython and libcython)
generic stepper class
fix step-into functions
have cygdb accept a '--' command line argument to disable automatic importing
replace gdb.execute() with something that actually captures all output
have 'cy break' break properly on line numbers

(run: python runtests.py Cython.Tests.TestStringIOTree \
                         Cython.Debugger.Tests.TestLibCython \
                         Cython.Compiler.Tests.TestParseTreeTransforms)

--HG--
rename : Cython/Debugger/cygdb.py => Cython/Debugger/Cygdb.py

Write cython_debug to the actual build directory (distutils and the cython
                                                  command line tool)
List --debug flag in cython's usage

Fixed small autocompletion bug

added the --pyrex-debug flag to Cython's build_ext
added the pyrex_debug boolean to Cython's Cython.Distutils.extension.Extension
    (for per-module debugging information)
debug output is written to the cython_debug directory
bin/cygdb is included (start this from the build directory)
    working commands: cy import, cy locals, cy break
when debugging is active, export all functions as extern