export indexeddb, handle missing keyRange object.

Not perfect, but good enough until real patches land
close #38 close #39.  Clear db, rather than dropping.

- modify "clear" to clear the db, rather than dropping.
- add new hook "drop" for when the "dropdb" behaviour is wanted.

(Consolidates code from mozkeeler/clearForReuse)
added dkeeler as contributor
version 0.10.0

- clear vs. drop
- (temporary) wrap of keyRange