Add an API to Track Package Requirements

Many applets need specific python packages in order to run correctly. Instead of requiring all of those packages at install time, applets should be able to install those packages at runtime. The API belongs in UA-core and needs to support the following features:
- Allow applets to specify which packages they depend upon (preferably in a distribution-agnostic manner)
- Warn the user at runtime if the necessary packages are not installed.
- Allow users to install the necessary packages using Package Kit