test for implicit snapshot on readStream()

also add some basic snapshot stuff in the native layer, ready for
implementing in JS too.
added snapshot test case that I forgot to add...
work-around for dodgy slow-stream
reformat package.json
fixed snappy compression, added compression tests
close() callback existential check
added useBatch=false option on writeStream
indentification
fix up virtual methods & reformat
version bump @0.3.3
Merge tag 'v0.3.3'
added `compress` boolean on open()
leveldb 1.7
remove compile warnings for solaris
cleaner build for osx
allow JS Strings through to native layer

major speed boost here for all utf8 databases
revamped benchmark suite
added leveldb 1.7.0 source tarball
added no-compression benchmark engine
improved benchmarks