Example demonstrating branch pinning

[jspahrsummers]

This needs some exploration, but could probably be done by:

1. Implementing an ArbiterSelectedVersion corresponding to a branch, with either a fabricated semantic version or an approximate one based on version history, then including the branch information in the selected version's metadata.
2. Implementing a custom requirement which is only satisfied by a selected version with the correct branch name metadata.
3. Including all branch versions in createAvailableVersionsList()