Allowing decorators (@autojit) to be bound as instance methods

Without this pull, the following does not work:

    class A(object):
        @autojit
        def a(self, c):
            return c
    A().a()

It does not work because the NumbaWrapper is not a descriptor, and misses the binding cue.

Note that I did not add any tests, as I am having issues getting them to run; I would be happy to add some to this pull request with some help getting them to run:

(ipython)walt@mrrobot:~/pycon/ipython/numba$ ./runtests.py 
Traceback (most recent call last):
  File "./runtests.py", line 6, in <module>
    import numba
  File "/home/walt/pycon/ipython/numba/numba/__init__.py", line 15, in <module>
    from numba import utils, typesystem
  File "/home/walt/pycon/ipython/numba/numba/utils.py", line 11, in <module>
    from numba.typesystem.typemapper import NumbaTypeMapper
  File "/home/walt/pycon/ipython/numba/numba/typesystem/__init__.py", line 9, in <module>
    from .typemapper import *
  File "/home/walt/pycon/ipython/numba/numba/typesystem/typemapper.py", line 18, in <module>
    from numba import numbawrapper
ImportError: cannot import name numbawrapper
(ipython)walt@mrrobot:~/pycon/ipython/numba$