conda.install.symlink_conda: Support 'pip install --user conda'

Conda currently assumes a system-wide install:

    $ pip install --user conda
    $ command -v conda
    /home/me/.local/bin/conda
    $ conda create -n foo --yes python
    $ ls -l ~/envs/foo/bin/conda
    lrwxrwxrwx 1 ... /home/me/envs/foo/bin/conda -> /usr/bin/conda

This series adds a check for such broken symlinks, and sets up the symlinks to point to whatever versions of `conda` etc. it finds in the user's `PATH`.
On Mon, Oct 13, 2014 at 09:20:23AM -0700, Aaron Meurer wrote:
> > @@ -38,6 +38,10 @@
> >  import traceback
> >  import logging
> >  import shlex
> > +try:  # Python >= 3.3
> > +    from shutil import which
> > +except ImportError:  # Python >= 2.4, < 3.3 (where we prefer shutil.which)
> 
> Conda only supports 2.7, 3.3, and 3.4.

Does global Conda compatibility matter?  Presumably at some point
there will be 3.5 support, etc.  These comments just give the versions
for which this code is compatible.