Add Iterators and Snapshots

Improve safety of borrow checker with ffi objects

initial comparator

Fix a few memory leaks.  Add valgrind feature build.

Fix a few memory leaks.  Add valgrind feature for conditionally building a testable binary.  Get rid of unstable vector usage.

Add support for WriteBatch

Nightly 6.28.15 update.

Cosmetic changes, warning fixes

(*) remove using of some deprecated functions
(*) reduce size of unsafe blocks

Updated to compile with rustc 1.0.0.alpha.2