Add option to remove trailing slashes

[dustinfarris]

This PR adds a new option to remove trailing slashes from API URLs.

Example usage:

// in your app's config/environment.js

var ENV = {
  APP: {
    API_ADD_TRAILING_SLASHES: false
    ...
  }
};

[benkonrath]

Looks good. Thanks!

[g-cassie]

Did you get this working with coalesceFindIds? I ran into an issue with it before. See emberjs/data#2468

[dustinfarris]

@g-cassie i have not tested/integrated this with coalescing yet (which is disabled by default). It's on my radar.