Enforcing that haptics are generate on the main thread

[bengilroy]

Generating haptics on a background thread can cause crashes (problem briefly discussed in #6).

This issue is to discuss the following:

• Should it be responsibility of HapticGenerator to ensure that haptics are called on the main thread?
• If so, what are the potential solutions? Should it flag when it's being run on a non-main thread? Should it just enforce that it's run on a main thread by moving to the main thread if necessary (or perhaps non-silently using an assertionFailure)?