Added separate set_auto_scale() and set_auto_mask() methods.
Added unit tests for <var>.set_auto_mask() and <var>.set_auto_scale().
Added global set_auto...() methods including unit tests.
Merge branch 'master' into feature/separate-mask-and-scale

Conflicts:
	netCDF4.c
Merge pull request #269 from cmarquardt/feature/separate-mask-and-scale

Separate set_auto_scale() and set_auto_mask() methods