use !isOpen() and bail out early in all methods

There were different ways of checking the state
of the db before actually doing something:

1. Check if open and do something, else bail

if (isOpen()) {
  // Do stuff
} else {
  // Bail
}

2. Check if closed and bail, else do something

if (isClosed()) {
  // Bail
}

// Do stuff

I chose number 2 for all methods, but replaced isClosed()
with !isOpen(). The reasons for this are:

- Consistency
- Get rid of a level of indentation
- No longer internally dependent of the isClosed() method ->
  Easier to remove if we so choose.
Merge pull request #76 from rvagg/bail-out-early-in-methods

use !isOpen() and bail out early in all methods
extracted all binding code to leveldown project
use leveldown@0.0.1
unnecessary tmp-test file
updated for leveldown@0.1.0 api
version bump
simplify callback signature, close #38, close #39
bump version to 0.6.0-rc1
*Stream() -> create*Stream() closes #77
update for 0.6.0 release
version bump for @0.6.0
refactor keyEncoding and valueEncoding
Merge pull request #78 from rvagg/refactor-encodings

refactor keyEncoding and valueEncoding
refactor options for db

move getOptions() into closure -> we can remove the this._options parameter

remove options variable from methods, use parameter instead
this is safe since we always create a new object and extend it
Merge pull request #79 from rvagg/refactor-options

refactor options for db
extracted ReadStream state manager, fixes #82
Merge pull request #84 from rvagg/multi-read-stream-close

extracted ReadStream state manager, fixes #82
changelog & version bump for @0.6.1
use xtend for options