Option to disable fine-grained location tracking

Sometimes fine-grained location (lat and long from the GPS) are
not needed and introduce privacy concerns.

Add option to enable/disable location tracking. Set to
true to keep same default behaviour as before.

Method names following the iOS api.
Move locationListening property to DeviceInfo
Remove "static" from getMostRecentLocation and locationListening

Location listening is an attribute of the deviceInfo object. It
means it can only be adjusted after initializing amplitude.
Before that deviceInfo object does not exist.
No need to pass context to getMostRecentLocation

Can use the context stored in the instance
Merge pull request #15 from ifrade/location-tracking

Option to disable fine-grained location tracking