Added pyenv-register command

One requested a function to install python via apt in issue #138.
I would like to suggest `pyenv-register` command helps to use python via apt (or other package manager).

usage:

    sudo apt-get install python-2.7
    pyenv register /usr/bin/python2.7

Then `$PYENV_ROOT/versions/system-2.7.*` will be created.

Q. Why this command is necessary?
A. `pyenv install` takes long time for compiling. In case of `apt-get` and `pyenv register`, compiling is not needed.

Q. You can use virtualenv to create `$PYENV_ROOT/versions/system-2.7.*`
A. `pyenv register` automatically download `virtualenv.py` if virtualenv is not installed.

Q.  `system-2.7.*` is ugly name.
A. `pyenv register` accept name as 2nd argument. However, I want more good naming.