Support executing binstubs from user gem install location

This brings in the functionality of [user-gems plugin](https://github.com/mislav/rbenv-user-gems).

Previously, when you did `gem install --user-install lolcat`, the "lolcat" executable will neither be found while rehashing nor available for execution. This confused users and made rbenv look like it was broken.

Caveats:

- `rbenv whence` isn't aware of user install locations
- User install locations for JRuby and Rubinius default to their default modes, i.e. this lookup isn't aware of `JRUBY_OPTS=--1.8` or `RBXOPT=-X19` or `--default-version` with which Rubinius was configured.

Fixes #554, fixes #465

/cc @sstephenson