Monitor and improve battery usage

[piuholo]

Information is required about the battery impact of the application.
This issue is related to issues #39 and #9 as the beacon detection and running background tasks could affect it significantly.

[piuholo]

Due to the behaviour of beacon ranging in connection to application state being so irregular on Android, depending on software version / device, I'd suggest the following:

• Start ranging all regions as soon as we enter one (regionDidEnter event by monitoring)
• Stop ranging all regions after we have exited all of the regions.

This way we could make sure that the device would use bluetooth only when necessary, as some devices do not stop the ranging even when the device is locked.

This would naturally require that we manage to get the regionDidEnter and regionDidExit events working reliably on all devices (See issue #39)