add support for custom alias names for versions

This patch allows you to specify an alias name when installing a python version, by adding a `-a`/`--alias` flag to the `python-install` script. This will allow you to install multiple copies of the same python version.

This is useful if you want to install multiple versions of 2.7.5 with different compile/configure options.

For example:
```
# the following line builds a 2.7.5 python framework with ucs2 on osx:
PYTHON_CONFIGURE_OPTS="--enable-framework --enable-unicode=ucs2" pyenv install -a 2.7.5 2.7.5_framework_ucs2

# and this will give us a framework with ucs4:
PYTHON_CONFIGURE_OPTS="--enable-framework --enable-unicode=ucs4" pyenv install -a 2.7.5 2.7.5_framework_ucs4

# and a regular 'shared' python, no framework:
PYTHON_CONFIGURE_OPTS="--enable-shared" pyenv install -a 2.7.5 2.7.5_shared
```

I believe that this minor patch might be useful for people trying to test tools and applications against different instances of the same python version.