Support Google Compute Engine

[stephenplusplus]

Continuing from #721

This PR puts a comb over the great work @mziccard gave us when he implemented support for Compute Engine. That includes various small documentation tweaks, code style things, unit tests, and system tests.

To Dos

• Documentation
  • Callback parameters
  • Upstream API links (depends on docs: upstream links #790)
    • Check for misspellings of "@"resource or improper {"@"link} tags
    • Confirm using v1 and not latest
• Tests
  • Unit
    • Address
    • Disk
    • Firewall
    • Index
    • Network
    • Operation
    • Region
    • Snapshot
    • VM
    • Zone
  • System

Changed since #721:

• Added an operation.onComplete method. Credit to Marco for the idea -- we couldn't think of a way to implement it un-confusingly at the time, so I hope this is a good solution.
• Removed all throwing where we were validating what the API expects. We typically let the API request go through and have its error clarify what was incorrect about the input. We only throw when our code cannot proceed.
• Added sane defaults in zone.createVM. Explained here. That necessitated two new modules: gce-images and to handle auth the same as gcloud does, google-auto-auth (this one can eventually be used in gcloud directly).
• Renamed Instance to VM: zone.createVM, zone.getVMs, etc.
• Added compute.getZones() and compute.getRegions()

Docs preview: https://stephenplusplus.github.io/gcloud-node/#/docs/master/compute

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[stephenplusplus]

Anyone interested, check out the system tests for an actual working sample of code!

[stephenplusplus]

@callmehiphop if you want to, take a look at the system tests (and the code) if you catch anything weird. It's about time to lock it down with unit tests.

Documentation will be a work in progress all the way up until the end.

[callmehiphop]

Sweet, I'll run through those this afternoon :D

[stephenplusplus]

Just for crosslinkability: https://groups.google.com/forum/#!topic/gce-discussion/mYwoVHa-mdQ

[stephenplusplus]

@mziccard we're getting close! I plan to include two commits when the PR merges, your original one and then all of our tests and such smashed into one more on top of it. Our CI bot is asking that you give us some type of 👌

[mziccard]

You have my permission to merge the commits :)

[stephenplusplus]

Phew!

[stephenplusplus]

Thanks everyone! This is now one of my favorites :)