Make re-building the API docs an optional part of gulp check-deploy task

[naomiblack]

Since we only re-build the API docs when there's a new release, it would greatly speed up site deploy if we could break those out and make them optional. I'm imagining something like:

• run check-deploy
• if check-deploy finds generated API docs in the local client are current with the checked-out local repositories for angular and angular-dart, skip re-generating API docs
• if the local repositories are newer, remove old API docs under angular.io and generate new ones
• on completion print a status message to stdio with the version that API docs were generated at

it would be a good idea to include some way to force generating the API docs (eg no version change, but testing a small tweak) but make the default be to not generate them.

[kwalrath]

This capability would also help if we decide that generating the dartdocs on Linux is necessary.

[chalin]

To regenerate the site after only a few minor changes to .jade or Harp data files, try using the --fast command line argument.

[chalin]

For the record: currently a check-deploy takes ~18 mins on my laptop, and ~15 with --fast, so that isn't much of a savings, but is 15 min. Is that too long? To reduce that further would involve performing the harp compilation itself piecewise.

[chalin]

Fixed by #2202.