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
Removes .keep file
Writes api::users#create action

[#18]
Writes the 'sign_up' route using ember form helpers
Removes more .gitkeep files
Updates controllers from @killpack's changes
Adds a link from the log_in path to the sign_up path
Add some asterisks to denote "required" fields