Added walker cleandb command

Because we currently don't keep track of live crawlers or automatically
expire claimed domains, added a 'cleandb' command to reset things if we
needed to kill the fetch manager off.
Removed cleandb command from cmd, will go in util
Merge branch 'master' into cleandb

Conflicts:
	cassandra/datastore.go
	cassandra/datastore_test.go
added util directory with cleandb command
datastore test: fixed old call to get test db handle
Merge pull request #36 from iParadigms/cleandb

Added walker cleandb command
Merge branch 'master' into IfModifiedSince