better error handling
easier to tweak mapping
change default bucket name to bleve
changed close of read only tx to Rollback from Commit

i was seeing deadlocks before this change
using Rollback to close read only tx is what the
built-in View() impl does, so i think its safe
fix two missing Close calls holding iterators open
added build tag 'leveldb' to enable this kv store

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