use GitHub API to download archive in install_github

- always use package name for naming downloaded file in install_github

- include tests

- bump httr dependency; don't explicitly set user agent
    - also affects one other GET call
Merge branch '466-github-api' of github.com:krlmlr/devtools into krlmlr-466-github-api
Merge branch 'krlmlr-466-github-api'
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