Bring up build infrastructure

[jpd236]

Includes the following components:

• Clean up the makefiles (perhaps just use Gradle for everything instead of Maven)
• Continuous integration builds/tests for commits and pull requests
• Other useful things to check in CI (e.g. error-prone, google-java-format)
• Artifact deployment (snapshots on commit, official releases on tags)

[jpd236]

Forked off separate bugs for google-java-format and error-prone; these don't need to block the next release.

The only remaining item we need for the next release is ensuring we can promote snapshot builds from OJO to Bintray, including GPG signing, which I'm in the process of investigating.

[jpd236]

Unfortunately I'm a bit blocked on GPG signing as I haven't gotten any answers at https://stackoverflow.com/questions/46480065/gpg-signing-snapshots-on-build-promotion-to-bintray around how to GPG sign SNAPSHOT artifacts and/or specify our GPG passphrase when promoting a SNAPSHOT to Bintray. And I really don't want to be publishing an official Volley release from my workstation directly. Currently I'm trying to poke around to see who can answer these questions.

[jpd236]

Per JFrog, OJO doesn't support GPG signing for SNAPSHOT artifacts or signing at promotion time via snapshotsToBintray. However, we should be able to sign them manually after the fact. There'll be a brief window where they are not signed, but if all goes well, not much of one.

Will take a shot at this soon.