Add Heading Support to SDK

[msmollin]

After some internal discussion around #389 - we landed on enabling proper heading support for the SDK. This will aid us in future implementations for Turn by Turn support, as well as allowing 3rd party developers to further customize the implementation for filtering.

General tasks we would need to get done:

• Expose filtering options to the underlying Location Manager
• Expose new API in delegate callbacks for heading updates (maybe also include block-based callbacks)
• Discuss any UI additions we can add for this (cone that indicates device's current orientation?)

cc @sarahlensing / @matteblair