add support for listing Force.com API versions

Almost all interactions with the Force.com API use a version number, for example `services/data/v29.0/limits`

That v29.0 is the version number.

This PR adds support for listing these version numbers and their associated names, like `Winter '11` - see the example below.

I envision a future where there will be another command, `force apiversions set 25.0`.  Then this client should perform interactions using that version (25.0).  Right now, `force.com` hard codes it to v20.0, which is really old.

Example:
    $ force apiversions list
    20.0 - Winter '11
    21.0 - Spring '11
    22.0 - Summer '11
    23.0 - Winter '12
    24.0 - Spring '12
    25.0 - Summer '12
    26.0 - Winter '13
    27.0 - Spring '13
    28.0 - Summer '13
    29.0 - Winter '14