Add support for io.js v3 (and therefore Node v4)

[lovell]

Placeholder to track the upgrade to the new version of io.js/V8 and all the nan changes this will require.

This requires comprehensive memory leak and Buffer performance checks.

[lovell]

V8 seems to have jumped on the monad bus with the (Haskell-inspired?) MaybeLocal type. This forces explicit exception handling, a good thing, but it does mean a fairly big code change to the marshalling of data between V8 and native C types.

[mahnunchik]

I've allowed failures in gulp-responsive for iojs v3 for a while: https://github.com/mahnunchik/gulp-responsive/blob/master/.travis.yml#L7-L9

[lovell]

The work-in-progress branch for this can be tested via npm install lovell/sharp#nan2

Everything compiles and the functional tests pass using io.js v3.1.0.

I have yet to run the leak checks (will probably require additional suppression rules for new V8 API) or used the benchmarks to measure the effects, if any, of recent changes to the Buffer object.

[lovell]

Leak checks run on Node.js v0.12.7, io.js v2.5.0 and v3.1.0.

Changes were required - see 4e67a50

[lovell]

Benchmarks run using Node.js v0.12.7 and io.js v3.1.0.

There's no statistically significant performance impact - (de)compression I/O is still the bottleneck.

[lovell]

Support for io.js v3 now in master - this will be published to npm as part of v0.11.2.