add support for prebuild to build for all abi version of node/iojs

[ralphtheninja]

While I was at it, I created this PR as well since ws use this native module as well as bufferutil, see websockets/bufferutil#12

[kleetus]

It would great if we can get this merged.

[paulirish]

this would help me out massively

[ralphtheninja]

If you decide to take it, we should update list of abi's as well, since both node 4 and 5 was released after this PR was made. See leveldown.

[ralphtheninja]

Updated PR to do prebuild --install rather than prebuild --download. And also changed prebuild npm script to do prebuild --all --strip, which builds for all abi versions.

I noticed bitpay added it to their fork