Support auto-detected installed Rubies (issue #29).

* Search in /usr/local/rubies, /opt/rubies, ~/.rubies.
* Only allow directories which contain bin/ruby.
Removed /usr/rubies and /usr/local/rubies to comply with FHS (issue #36)
Added a crude prototype of chruby-exec (issue #24).
Added another check for no arguments.
Added tests for chruby-exec.
exporting and Array does not work.
Changed chruby-exec to exec $SHELL.
Make that branching logic more clear.
Intial prototype of auto-switching for chruby (issue #40).
Update share/chruby/auto.sh

make sure to preserve `cd` return status and no fail on `set -e`
Merge pull request #41 from mpapis/patch-3

Update share/chruby/auto.sh
Style.
Use precmd_functions/PROMPT_COMMAND in auto.sh (issue #40).

* This has the added benefit of detecting a new .ruby-version after a
  `git pull` or `echo "1.9" > .ruby-version`.
Update share/chruby/auto.sh

fix precmd quoting issue ... or just simplify code
Merge pull request #45 from mpapis/patch-1

Update share/chruby/auto.sh
Merge branch 'auto-detection' into 0.3.0
Fixed typo.
Added tests for chruby_auto (issue #40).

* Due to PROMPT_COMMAND and precmd not being called in non-interactive
  mode, I had to explicitly call chruby_auto after every cd invocation.
Added a test for explicitly not entering the project dir (issue #40).
No need for a for-loop if we're only checking two directories.