Support binary keys

[vweevers]

• Have the abstract-leveldown test suite assume binary keys are supported (remove process.browser checks) (get rid of process.browser checks abstract-leveldown#121) Remove process.browser checks and move binary key tests to isolated test file(s) that can be skipped by implementations.
• Or until then, add a custom test to level-js
• Assume use of modern browserify (where Buffer is a subclass of Uint8Array)
• Feature-detect binary key support
• Update to latest abstract-leveldown and implement _serializeKey as (simplified):

var supportsBinaryKeys = true;

Level.prototype._serializeKey = function (key) {
  return !supportsBinaryKeys && this._isBuffer(key)
    ? String(key)
    : key
}

This means the test suite can pass in browsers that support binary keys as well as those that don't (IE, Edge, Chrome 49).