Drop the separate support for Node buffers

Currently jDataView has ≈45 lines of code dedicated to a special support for Node buffers.

Such support is not necessary, because e.g. `new DataView(new Buffer(1))` is possible in Node since version 0.5.6.

(Probably also in Node v0.5.5, but I could not test that version on my Windows XP, it crashes.)

This pull request creates a DataView out of a Node buffer, and then `this._isDataView` and `this._isNodeBuffer` situations are treated uniformly.

This pull request also fixes a couple of typos and puts `this._start = byteOffset` inside an `else { … }` clause (it probably fixes a bug).