README: mention https://github.com/yyuu/pyenv-pip-rehash

doc: use recommended python versions

Import changes from ruby-build v20141028

Warn about #264 in the README

Add Miniconda/Miniconda3 3.7.0 and Anaconda/Anaconda3 2.1.0

SHA256 values generated with direct downloads from repo.continuum.io

Allow installing CPython with `make altinstall` instead of
`make install` with following command-line.

```sh
$ env PYTHON_MAKE_INSTALL_PACKAGE="altinstall" pyenv install -v 3.4.2
```

Install script of setuptools-6.1/pip-1.5.6 will create those links if needed

Add Makefile for 'make test'

This was introduced when merging the rbenv upstream change
(https://github.com/sstephenson/rbenv/commit/6bb7f07d2d).

Optimize pyenv-which: implement remove_from_path in Bash

This is the remaining part of
c69d9a1128.

    commit c69d9a1128
    Author: Mislav Marohnić <mislav.marohnic@gmail.com>
    Date:   Mon Oct 13 12:39:47 2014 +0200

        Isolate rbenv-which tests from any `.ruby-version` file on the system

        Having a `.ruby-version` file in any of the parent directories of the
        local clone of rbenv could cause the test suite to fail because it
        wasn't expecting a local version to be set.

This adds the missing parts from the upstream rbenv merge:
f4652fbbf0

- e4cbf04592
- 3ee395f9b5

Do not quote completions in $reply

pyenv-version-file-write: do not use rm, but clobber the file

With e.g. /usr/local/bin/.python-version owned by some user, `pyenv
local foo` would fail, if the user has no permissions for
`/usr/local/bin`, but only the `.python-version` file.

With an empty list of completions, zsh would complete a space / advance
the cursor by a space when completing.

doc: note about using multiple versions

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