by default we now use the pure go boltdb kv store
it is less tested at this point but appears to work
test pass, and moves us closer to the goal of being
able to just "go get" bleve
New is now used to create new indexes
Open is used to open existing indexes
calls to Open no longer specify a mapping because the mapping
is serialized and stored along with the index
now can track array positions for field values
stored fields now include this in the key
and the back index now uses protobufs to simplify serialization
closes#73
