This makes it possible to execute rbenv shims without rbenv's bin
directory in the path.

help system where each command holds its own docs

Before, lines of help that were indented were stripped away.

A command doesn't have to specify Usage docs if it doesn't accept any
arguments. The default usage for a command will be printed as:

    Usage: rbenv ${command}

System ruby is already on PATH (that's the definition of system ruby) and by
duplicating its path by putting it in front, we can break the user's
environment.

Fixes #275

Automatic local exec

Fixes #119

Fixes #294

This is to insure portability to ksh.

Fixes #205, references #209

Docs are comprised from "Usage", "Summary" and "Help" sections, where
"Help" can span multiple commented lines. If it is missing, "Summary" is
shown in its place.

References #204, references #206

Should be `/usr` or `/usr/local`, not `/usr/bin` or `/usr/local/bin`

Fixes #274

Closes #263

Don't have `command -v` abort the whole script prematurely.

Use `rbenv-commands` instead of `rbenv commands`. The latter
unnecessarily goes through `rbenv` executable.

Fixes #254

fix `versions` in case current version doesn't exist

./configure doesn't exist by default in repo: use autoconf to create it.

Gets rid of `doc/README.mdtoc` and its build script. Since GitHub.com
renders anchors for each heading, all we have to do is put a simple
table of contents into `README.md` itself, and everything will get
linked up nicely.

Pros of this approach:
* We don't have to point out to people not to edit `README.md` anymore
* We don't have to run the build script each time README gets edited

Cons of this change:
* The "chapter" numbers are lost. They were silly anyway.

`doc/mdtoc` renders a Markdown table of contents for a Markdown file.
`doc/filter-toc` filters that down to only headings after ToC.
This script can be used to easily insert ToC into the current document
when editing `README.md` with, e.g., Vim:

    :read !doc/filter-toc %

fix install instructions for ubuntu & other linux

fix obsolete usage instruction

`rbenv-versions` tries to read the current version to display a marker
next to it, but if that fails the whole script aborts.

This change makes it so that the failures from `rbenv-version-name` are
tolerated. It also makes the `--bare` mode never call it in the first
place, because it doesn't need to display a marker.

Fixed broken link to RVM in documentation

Added in short note on Uninstalling Rubies under new 2.4 section in README

All the changes to documentation should be done in doc/README.mdtoc

Just trying to keep the README.md up to date with Ruby versions