Switch to lerna

[KevinGrandon]

We currently have a hacky monorepo implementation, instead we should switch to something like lerna.

ghostjs-core -> packages/ghostjs-core
ghostjs-examples -> packages/ghostjs-examples
ghostjs-website -> packages/ghostjs-website

[KevinGrandon]

We also might want to split up the current ghost examples package to be something like:
packages/ghostjs-test-cases
packages/ghostjs-tests-slimer
packages/ghostjs-tests-phantom
packages/ghostjs-tests-electron