Add dispose method

[Mecharyry]

Problem

We are currently leaking Merlin and any associated callbacks because we do not safely remove references to context when destroying an activity.

Potential Solution

Investigate the demo application for memory leaks and attempt to fix them. This may or may not involve adding a dispose method to Merlin to ensure that all references are destroyed.

Impact

We no longer leak Merlin and the client does not explicitly need to set Merlin to null when destroying an activity.

[Mecharyry]

Addressed and released as part of v1.1.0.