[Feature] Add SwiftPackageManager compatibility

[RemiRodrigues]

Hello,

This is not an issue but a feature proposal.

In order to import PromiseKit/CoreLocation as the same way of PromiseKit and PMKAlamofire, it would be nice that PromiseKit/CoreLocation can be imported using SwiftPackageManager.

[mxcl]

Trivial pr. Feel free!

[Overcot]

#31 maybe close this issue because of this pr?