Add testing, linting, and building

[samtstern]

@nicolasgarnier would you be open to adding something like this:
firebase/snippets-node#11

It's a few parts:

• Use lerna to build and test each module
• Lint all source files against a single eslint config to ensure correctness and consistency

If so I'd be happy to implement / assist implementation.

[nicolasgarnier]

@samtstern Hey! Sure that would be nice :)

Some notes:

• There is not a lot of tests unfortunately, the only tested sample is https://github.com/firebase/functions-samples/tree/master/quickstarts/uppercase/functions
• There is now have an ESlint config file inside each individual sample functions directory. This is in order for enforce checking styling on deploy (The plan is to add a presubmit rule to check styling on Functions deploy). We added a rule that forces having a return statement inside all Promises. We did this because one of the main mistakes developers do is forgetting to chain the Promises in their code and this should help.