Use GitHub API v4, add options and improve trust algorithm

[Ullaakut]

Goal of this PR

This PR is a rewrite of 90% of Astronomer's code. It includes:

• Using the GitHub v4 API in order to make fewer requests and get only the data that matters to astronomer
  • This increases scanning speed between 400% and 4500% compared to the 0.1.0 version.
• Improves the trust algorithm computation
  • Private contributions count about twice less than other contributions for computing the overall trust
  • Issues, PRs, code reviews, commits and repository creations also have different weights when computing the overall trust
  • All percentiles from 5 to 95 are now computed when the --details flag is enabled
• Adds two options
  • -d, --details which makes Astronomer print more detailed statistics
  • -c, --cachedir which allows users to specify a custom cache directory in which to store cache data. It defaults to ./data.
• Update documentation
  • Update screenshots to use the latest version
  • Document new options
  • Document go install procedure
  • Update FAQ
  • Thank Renee French for the Go Gopher

Fixes #4
Fixes #6
Fixes #7

How to test it

• docker build -t astronomer . should work
• go build -o $GOPATH/bin/astronomer *.go should work
• Run astronomer scans of small repositories using both the Docker image and the binary, playing around with the new options
• If you can, run an astronomer scan on a big repository