beta 3 release

* list cmd to show available versions, marking the currently active version (#25)
* status cmd to print current ruby version and source info (#24)
* include the release in the local install dir name (#22)
* `-f` option to specify version files to use (#13)
* reset the env when activating from the `rb` cli (#9)
* remove all whitespace when reading version string from files (#16)
* added a changelog (#15)