More JavaScript-like API

[mikepb]

I'm satisfied with the stability of the new API:

• CoffeeScript is commented, but if you prefer JS, do cake build:coffee to compile to JS. It more or less reads just as well.
• I worked hard to not have memory leaks, but I don't know if this is completely true.
• Tests don't test options, but do test everything else, for now.
• Not production tested!

There are also some caveats from the requirements of the underlying leveldb library:

• Batch objects are read-locked when committing, meaning that batch.write() (async) with concurrent batch.put() or batch.del() will throw an error.
• Iterator objects support only one operation at a time, meaning that running two methods concurrently (async) will throw an error.
• There is an advanced option on the native iterator object iterator.self.refSlice = true to return buffers which ref the returned leveldb Slice instances instead of copying the data into a new buffer. Doesn't work without the as_buffer option for iterator read operations. The caveat is that the buffer is only valid until the next seek and may crash the program if used after seek. Use with care.

Cheers.

[mikepb]

Some thoughts in hindsight:

• Supporting sync operations made the code overly complicated. Are any of them necessary?
• Should probably use ref Buffers for everything to avoid copying data repeatedly. handle.get() and batch.del() already do this for keys; batch.put() does this for both the key and value; there's the iterator.refSlice = true option that makes this true for keys and values returned from the iterator as well.

[mikepb]

Will remove sync functions to simplify C++ code, other improvements before merging.