Add detail regarding activating multiple Python2 and Python3 versions simultaneously.

Closes #699, fixes #666 [ci skip]

Cfr. #688

The `rbenv versions` info highlights 1.9.3-p327 as the current version, and
making `rbenv version` sample output match that helps make it clearer.

The license information is already present in the LICENSE file.

We won't maintain it anymore. Instead, the changelogs can be obtained
from the project's Releases page: https://github.com/sstephenson/rbenv/releases

git:// protocol is blocked by a lot of servers, https works fine.

Made it consistent with the rbenv documentation by having the cloning step in the installation clone it directly into the home directory.

Add a note about how to activate/use multiple versions to the main
README, to the "Choosing the Python Version" section.

This gives users an idea of which versions of python are available. Other version managers ([nvm](https://github.com/creationix/nvm), [rvm](https://rvm.io/rubies/list), etc) tell users how to list available versions as well.

Provide an `rbenv install` example and explanation lower down, so that
both folk that are following git install and Homebrew install will
finish at the same section and learn how to use ruby-build.

Also remove step 6 of manual install process: `rbenv rehash`. It's
unnecessary at that point, even if they already did `rbenv install`
(which rehashes automatically).

Fixes #455, closes #456

It can be confusing if people mistake the example as instructions to
install ruby-build with.

Don't suggest that they add to their `.profile` anymore because:

1. They would have to restart the desktop session for their change to be
   reflected, or source the file manually;
2. An error in `.profile` may prevent logging in;
3. The `rbenv()` shell function and shell completions won't be available
   in Terminal since it doesn't start bash in login mode by default.

Therefore, suggest that they use `.bashrc` instead. This will be
immediately reflected in a new Terminal tab. If bash is started in login
mode somehow, the default `.profile` is set up to source `.bashrc` anyway.

Also, don't suggest restarting the shell environment with
`exec $SHELL -l`, since we don't know what was the original mode that
their shell was started in. (OS X Terminal.app will be login mode,
Ubuntu Terminal has non-login mode by default.) Mode can be checked with:

* bash: `shopt -q login_shell`
* zsh: `[[ $options[login] = "on" ]]`

But since this is gnarly, let's just avoid it altogether and go the
easy route.

Closes #305, fixes #373, reverts #286

Updated README.md contents so that installation instructions cover
not only the common case for installation directory, i.e. `~/.pyenv`
but any other at the user's choice.

Closes #432 [ci skip]