Python: Enable MASS unit in FloatProperty

MASS unit was already implemented for the C api. Only making sure it is
accessible in the python api. Also added 'CAMERA' to the documentation as a valid option.

source/blender/makesrna/intern/rna_rna.c

@@ -121,6 +121,7 @@ const EnumPropertyItem rna_enum_property_unit_items[] = {
 	{PROP_UNIT_TIME, "TIME", 0, "Time", ""},
 	{PROP_UNIT_VELOCITY, "VELOCITY", 0, "Velocity", ""},
 	{PROP_UNIT_ACCELERATION, "ACCELERATION", 0, "Acceleration", ""},
+	{PROP_UNIT_MASS, "MASS", 0, "Mass", ""},
 	{PROP_UNIT_CAMERA, "CAMERA", 0, "Camera", ""},
 	{0, NULL, 0, NULL, NULL}
 };

source/blender/python/intern/bpy_props.c

@@ -1904,7 +1904,7 @@ static void bpy_prop_callback_assign_enum(struct PropertyRNA *prop, PyObject *ge
 "   :type description: string\n" \
 
 #define BPY_PROPDEF_UNIT_DOC \
-"   :arg unit: Enumerator in ['NONE', 'LENGTH', 'AREA', 'VOLUME', 'ROTATION', 'TIME', 'VELOCITY', 'ACCELERATION'].\n" \
+"   :arg unit: Enumerator in ['NONE', 'LENGTH', 'AREA', 'VOLUME', 'ROTATION', 'TIME', 'VELOCITY', 'ACCELERATION', 'MASS', 'CAMERA'].\n" \
 "   :type unit: string\n"	\
 
 #define BPY_PROPDEF_NUM_MIN_DOC \