foundation for a help system where each command holds its own docs

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
Merge branch 'master' into help
Improve syntax for inline documentation and allow for multi-line usage
Add `rbenv help --usage`
Don't print version before help summary (bare `rbenv` already does this)
display help for commands that have Summary but not Usage

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}
allow indented lines in help text

Before, lines of help that were indented were stripped away.
new-style documentation for most commands
Wrap documentation comments at 70 columns
Documentation tweaks
Merge pull request #292 from sstephenson/help

help system where each command holds its own docs
Include `install` and `uninstall` in help if ruby-build is installed
Usage consistency
Prefer .ruby-version to .rbenv-version for reading local versions
Accept "ruby-" version prefix but print a warning to stderr
Read both .ruby-version and .rbenv-version, but write (and migrate to) .ruby-version
Tweak the ruby- prefix warning
Document .ruby-version
Shims include the full path to rbenv

This makes it possible to execute rbenv shims without rbenv's bin
directory in the path.
Clarify that .rbenv-version is removed when migrating