refactored around setting up `$PATH`

* use `${RBENV_ROOT}/shims` as `$PATH`.
  this is the standard way to execute commands installed in rbenv
* run `rbenv rehash` properly after `gem install` to update binstubs
* respect `${RBENV_ROOT}` if it has been defined in shell (#1)
* escape command line arguments
* added `run` and `capture` methods
Merge pull request #15 from yyuu/use-shims

Please use `${RBENV_ROOT}/shims` as `$PATH` of build environment