Migrate integration tests to Spectron

[GabeIsman]

I didn't know about this project, but it looks really good. https://github.com/electron/spectron. It's much cleaner than the current setup for a couple of reasons:

1. It would allow us to run integration tests directly on the built binaries.
2. It would allow us to access real electron APIs from the tests, so we can stop mocking electron.remote and get more test coverage around the OS interactions.
3. It would solve the integration test issue in #34, by allowing us to directly access electron.clipboard.read from the test.

[garrettr]

39 is ready for review (I requested one from @GabeIsman) and will resolve this issue. Further work is required to realize all of the potential benefits of migrating to Spectron, as listed in the description, so I think we should file follow-up issues to:

• Run integration tests directly on the built binaries
• Stop mocking electron.remote
• Increase test coverage of OS interactions