Merge rbenv to have a common ancestor for future merges

Details on multiple Python2 and Python3 versions.

Add detail regarding activating multiple Python2 and Python3 versions simultaneously.

This merges
4d72eefffc
to build a common ancestor for future merges.

This is branched off f48a5b11d709ec616f766864b8e0ab894cb30962, which was
the last manual merge.

Discussion / initial idea: https://github.com/yyuu/pyenv/pull/286#issuecomment-66565475

This was done using:

    # Keep our changes for "unmerged, both added"
    for i in $(git status --porcelain | grep '^AA ' | cut -d\  -f2); do
      git checkout --ours $i
      git add $i
    done

    # "git mv" rbenv files to our name, keeping the current contents.
    for i in $(git status --porcelain | grep '^A ' | sed 's/^A  //'); do
      ours=${i//rbenv/pyenv}
      test -f $ours || { echo "Skipping: $i"; continue; }
      git mv -f $i $ours
      git reset HEAD $ours
    done

I've handled the following then manually:

    - rbenv.d/exec/gem-rehash.bash
    - rbenv.d/exec/gem-rehash/rubygems_plugin.rb

This should allow to merge rbenv in the future using:

    git merge rbenv/master -s recursive -X rename-threshold=5%

I am not sure about the rename-threshold, 25% also worked for one file
I've tested.

Conflicts:
	.gitignore
	.travis.yml
	LICENSE
	README.md
	src/Makefile.in
	test/--version.bats
	test/commands.bats
	test/completions.bats
	test/exec.bats
	test/global.bats
	test/help.bats
	test/hooks.bats
	test/init.bats
	test/local.bats
	test/prefix.bats
	test/rehash.bats
	test/run
	test/shell.bats
	test/shims.bats
	test/test_helper.bash
	test/version-file-read.bats
	test/version-file-write.bats
	test/version-file.bats
	test/version-name.bats
	test/version-origin.bats
	test/version.bats
	test/versions.bats
	test/whence.bats
	test/which.bats

2.7.9 - Stop '/Library/Python/X.X/site-packages' from being added to sys.path

src/configure: fix sed expression

This has been fixed for rbenv in
https://github.com/sstephenson/rbenv/commit/050f750.

http://fwierzbicki.blogspot.fi/2015/02/jython-27-beta4-released.html

add pypy-2.5.1 portable version

fix url and checksum for pypy-2.5.1-src

Fix require_distro

add python 3.5.0a3

fix pypy-2.5.0-src build directory

Add pypy-2.5.1

Closes #699, fixes #666 [ci skip]

Fix pattern replacement to allow flags with commas.

add python 3.5.0a2

Remove dead links to History and License from README

Cfr. #688

Add python3.5.0a1

Add python3.4.3

Remove history, license information from README