Add String() to the Field structure
Add the sentinel values for all db on creation

Fix #772
update the changelog
Fix some bugs with retention policy of shard spaces

This commit fixes two bugs:

Don't try to parse "inf" retention policy when creating a shard
space. This caused a panic to be thrown when a shard space is created
with infinity. Fix #774

`getExpiredShards()` used shard duration to determine which shards are
expired but should be using shard retention duration instead. Close #769
Fix travis
add a flag to force output to go to stdout
Non admin database users shouldn't be able to drop series

Fix #736
Allow 'null' to be supplied as fill value

Add to unit tests to test fill, including new "null" support.

Fix #713.
Dry the fill() tests
Merge branch 'pr-738'

Close #738. Fix #713
Fix a unit test that has changed in 91078c0

Non admin database users shouldn't be able to drop series. See #736 for
more details
Fix an integration test that has changed in 91078c0

Non admin database users shouldn't be able to drop series. See #736 for
more details
merge in upstream