This fixes issue: https://github.com/yyuu/pyenv-virtualenv/issues/36

It will be done by `get-pip.py` and we don't have to do anything explicitly.

First release after my son's birth....

Currently the link to venv points to the Japanese version of the Python documentation.
Change this link to the English version of the Python documentation.

It is necessary only if user want automatic fashion of activation.
If the users of pyenv-virtualenv doesn't need the feature, they can
omit the configuration safely.

Restart shell after installation of the plugin to enable `pyenv-sh-*`
commands.

Explicitly define the parameters that you pass into 'pyenv virtualenv' and what they stand for.

Make it explicit that the 'installing as a plugin' section is assuming that you've installed pyenv in the standard location and what to do if you've done something else.  Clarify the language a little bit about checking out a specific release vs. pulling the current development version.

`virtualenv` package into source version and then use it.