Remove unnecessary Long.signum
Fix for NPE when determining device resolution. Remove unnecessary device metrics (resolution, carrier, probably density at some time) from request JSON if they are empty.
Ability to disable session time updates
Update README.md
Merge commit 'afcd73b6a2d726c8fc9c8dc666f63b68a775d6bf'

* commit 'afcd73b6a2d726c8fc9c8dc666f63b68a775d6bf':
  Update README.md
  Ability to disable session time updates
  Fix for NPE when determining device resolution. Remove unnecessary device metrics (resolution, carrier, probably density at some time) from request JSON if they are empty.
  Remove unnecessary Long.signum
updated JavaDoc in DeviceInfo class; added comment about handling of NPE in DeviceInfo.getResolution; minor code formatting changes
update unit tests to reflect recent changes in DeviceInfo class