Ignore /global instead of /default
Support install locations other than $HOME/.rbenv.

Define RBENV_HOME env variable in libexec/rbenv and let all script delegates use this variable to determine rbenv's install location.
Adds error handling to rbenv-sh-shell
Adds sh-shell subcommand to unset RBENV_VERSION
Merge pull request #80 from lordzork/master

rbenv-sh-shell enhancements
reset isn't standard
Merge pull request #56 from mybuddymichael/gitignore

Ignore /global instead of /default
`rbenv-version-name` returns `system` for empty version files. Closes #62, #65, #77.
Deal with an empty result from expand_path
Merge pull request #81 from eric/empty_path_to_remove_in_which

Deal with an empty result from expand_path
Make rbenv-sh-shell consistent with rbenv-local
Fix quoting in rbenv function
Only shift if an argument is passed. Fixes #78.
Fix zsh autocompletion. Closes #47.
Don't leak local variables
Exit with 0 status when a version file is read successfully
Add `rbenv local --unset`
Merge remote-tracking branch 'alexrinass/systemwide'
RBENV_HOME -> RBENV_ROOT
Use RBENV_ROOT to search for rbenv-exec plugins