Add dogpile as a requirement
Remove empty line
Start integrating dogpile for caching

- Add a default dogpile cache backend in the default_config file
- Initialize the cache when starting the app
Catch exception when adding a package which break the DB constraint (ie: double name)
Add an extras.py file providing extra API endpoint for BZ sync and others

These endpoints are not directly related to the PkgDB role but are required by third
party tools.