Post release version bump 0.5.2 -> 0.5.3.
Convert int and double fields to string values so they can be sent to
the stat accumulator.
Merge pull request #860 from mozilla-services/stat_filter_numeric_values

Stat filter numeric values
Merge branch 'versions/0.5' into dev

Conflicts:
	CHANGES.txt
	CMakeLists.txt
	cmd/hekad/main.go
	docs/source/conf.py