Unwrap

[vweevers]

• [x] get, put, del
• [x] batch
• [x] iterator
• [x] open
• [x] close

Closes #46.

[vweevers]

Supersedes https://github.com/Level/level.js/pull/55 (you can now pass a prefix option to the contructor) and I think https://github.com/Level/level.js/pull/53 as well (it seems the use case there is the same: specify a custom prefix).

[vweevers]

This changes the default prefix and removes options that IDBWrapper had but we don't, so I'm adding the upgrading label.