Adds tools for generating indices locally and fetching remote repositories

This is a work in progress, but I pushed it so we can start iterating over the design. This branch adds two new commands to rocon_app:

* ```rocon_app index``` generate indices for Rapps in a local tree. The indices can then be committed to a repository to release the rapps.

* ```rocon_app add-repo URL``` retrieve a remote index and its associated rapps, you can test it with this branch:

```$ rocon_app add-repo https://raw.githubusercontent.com/esteve/rocon_app_platform/remote_index/rocon_apps/apps/talker.index```

```add-repo``` will fetch the index and store the Rapps into the ```rapps/``` directory in the current path.

Things to discuss:

* Where should Rapps be installed?
* How to handle versioning?
* Download Rapps individually or by package?
* Any other information that should be in the index file?

Resolves #189