WIP: add the ability to nest teams

[jacobwegner]

Adds the ability to nest Teams.

Remaining:

• Standardize convenience methods for ancestors and descendants.
• Document new templates and add to the appropriate starter project.

For further discussion:

• Should nesting be supported / the parent relation applied to SimpleTeam as well as Team? (see Split Team model functionality to abstract and concrete classes #25)
• Is there a better way to handle slug creation on child teams?
• Are there more performant ways to resolve get_root_team and ancestors using the ORM?
• Do we allow for "implicit" memberships of teams beneath a parent (see Team.for_user).
• Related to above, do we enforce a single membership for a user across a Team and its descendants/ancestors?

Comments/Gripes

• Slug building off of parent pks (versus parent slugs)
  • Why not use the slugs (database limitations)
• Prevent a team from being an ancestor
• Team methods should reference self

[jacobwegner]

cc: @jtauber and @gregnewman

[jacobwegner]

cf #2

[coveralls]

Coverage decreased (-1.5%) to 48.429% when pulling f25b1e4 on jacobwegner:add-nested-teams into 96a72ec on pinax:master.