Initial boilerplate for DataView

Wrote the initial boilerplate for DataView - wanted to create a pull request now, before going on implementing the actual set/get* methods (which are the core of the functionality).

The impl is based on TypedArray's, and is straightforward, but wanted you to take a look, just in case f* something up ;)

Stuff that remains to be done:

* set/get* methods (endianess handling, pretty much)
* Constructor handling of more arguments (not only ArrayBuffer)

Things to be address (maybe):

* Some stuff is handled in _specop_get() (as "length"), but other stuff is defined as an actual property (TypedArray's "byteLength", "byteOffset", etc). I used the latter for DV. Wondering about it.