Make setAnnotations' boolean parameter optional.

[kberg]

Please read the guide to making dygraphs changes:
http://dygraphs.com/changes.html

Pull Requests will only be accepted if:

• You clearly explain what you're adding and why you believe it's an
  improvement. For example: "Fixes issue Custom drawing circles #123".
• You adhere to the style of the rest of the dygraphs code base.
• You write an auto_test for the code that you're adding.

Be sure to document any new options you add. Also be aware that PRs which add
options are likely to be rejected. dygraphs already has many options. If you
can fit your feature into one of those or implement it as a plugin, it will be
more likely to get merged.

[coveralls]

Coverage decreased (-5.1%) to 85.06% when pulling b4ddb43 on kberg-patch-1 into aaec6de on master.