Tag GitHub v2.0.0

[jrevels]

obligatory link to package

A fair amount of breaking changes have been made since the previous tagged version of GitHub.jl (hence the bump in the major version number). A brief overview:

• v0.3 has been dropped in favor of v0.4.
• Webhook event handling infrastructure has been added (cc @tkelman since you previously expressed some interest in this)
• The package now has tests and tracks code coverage
• A large number of REST wrapper functions have been altered to make the API more consistent/flexible/predictable.
• A fair number of REST wrapper functions have been added to bring GitHub.jl a little bit closer to feature parity with the functionality that GitHub's API exposes (a lot of work is still up for grabs in this regard).

Getting this tagged will bring us one step closer to the resolution of JuliaLang/julia#13893 by providing a more robust interface to GitHub's CI-related features.