Upgrade to Nan 2.0

[springmeyer]

Started this in order to be ready to support io.js 3.0.0. It turned out to require a major overhaul so I pushed through using a combination of nodejs/nan#376 (comment) and a lot of manual fixes.

Thinking about merging this fairly soon because otherwise other pull requests are going to have to be remade a lot over time.

Still needs, before merging:

• confirm travis/appveyor passing
• remove now uneeded handlescopes (based on upgrade to NAN v2 Level/leveldown#195)
• convert all Handle -> Local (based on upgrade to NAN v2 Level/leveldown#195)
• add buffer requirement for constructing palettes to changelog
• try using the zero copy Nan::NewBuffer interface where possible (transfers ownership of char*) - Look for zero-copy optimizations using Nan::NewBuffer #531

[springmeyer]

plan on this: rebase once #505 is landed.

[ChALkeR]

Adding to the list: nodejs/node#2798.

[springmeyer]

Working on finishing this. Ran into #527 and #528 while trying to get to 100% coverage which is the last problem with the branch.

[springmeyer]

Now 100% on this branch again (https://coveralls.io/builds/3694404). Will merge once travis is green.

[springmeyer]

followup: #531