Jfw return value rodeo

[chardan]

Fix regression with riak_kv.

[slfritchie]

The basics look good. TravisCI's unit tests are passing.

One thing before final review: a second combing of the -specs in src/eleveldb.erl are in order.

1. Check that the return values from the async work items match. Jesse and I have already gone through some but not all.
2. Add the type for the "ER" returned when the async job submission fails. Perhaps add a -type async_failure() :: whatever_the_ER_thingie_looks_like() and then add async_failure() as an option to the return value spec for each of the async functions?