cassandra: use clear_column_family!
add exception class
cassandra: specify more robust default timeouts
add comment

[ci skip]
travis supports cancel
add option sync to Cache#load (See #56)
add more tests
add LMDB
lmdb add multiprocess support
use improved lmdb api
use lmdb gem
add rom adapter
Merge branch 'master' into staging

Conflicts:
	lib/moneta/adapters/cassandra.rb
	script/benchmarks