Pan transition

Added a new pan transition, see #28

Cube transition

Added a new cube transition, plus a couple of small bug fixes:

• #15
• #16

Portal transition

Added a new transition 'portal'

NatGeo transition and various bug-fixes

This release includes the following:

• A new transition modelled on the National Geographic app #10

The following bug fixes and improvements

• #7 explode transition performance improvements
• #4 app no longer hangs if you complete a manual transitions
• ... plus some other small changes.