Add requisite properties to packages.
Clean up package_attrs with a little metaprogramming.
Install pre- and post-reqs in #install!.
Add pre- and post-reqs to B9 and Astronomer's as a proof-of-concept.
Fix grammar in #install output.
Make package_attr methods available to instances and classes.
Refactor Package.

Remove some unnecessary calls to self.class and DRYed out the code a bit.
Add some aliases for the requiste methods.
Merge pull request #263 from ucarion/requisites

Requisites