report error if there's a nil storage driver
Merge pull request #63 from monnand/always-has-a-storage-driver

report error if there's a nil storage driver
Merge pull request #64 from monnand/master

unit test for memory storage driver
storage unit test percentiles
fix a bug in memory storage and add a new unit test
add unit test
defines the order of the returned recent stats
add doc
remove constraints
docs
do not always allocate a new slice
check error on AddStats()
fix a deadlock bug along with unit tests
increase the coverage
defer unlock
fix typo
stats returned in chronological order
comment.
Merge pull request #65 from monnand/storage-unit-tests-percentiles

More unit tests for storage drivers and a bug fix for memory storage