Add friends endpoint

[#34]
Add friend model

I tried to stick this in its own file, but the load order was such that
friend.js got loaded before user.js, causing a reference error. Rather
than futzing around with load order, I opted to just stick the friend
model at the end of user.js.

[#34]
Add friends route and template

[#34]
Add friends link to sidebar

[#34]
Merge pull request #35 from gschorkopf/killpack/friends

Killpack/friends