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`.
Conda only supports 2.7, 3.3, and 3.4.