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Restore the ability for plugins to set Store headers

The Auth plugin in the v1.2.x series relies on the Store header parsing
the value of a jQuery `annotator:headers` field on the annotation
element in order to set the auth token header on requests to the store.

This functionality got lost in the refactoring of the Store plugin,
which I don't believe is a bad thing (for one, the implementation fell
far short of the "Tell, Don't Ask"[1] design philosophy).

We still need a way to get headers into the Store plugin, at least for
now, so this commit adds a `setHeader` method to the Store plugin, and
updates the Auth plugin to use that.

[1]: http://pragprog.com/articles/tell-dont-ask
Add simple utility function to check if two sets are equal
Hover: update content when moving over a new highlight

Changed hover behavior so that when the mouse moves over a new highlight,
the annotations displayed in the hover are updated accordingly.

Fixes #222.
Improve comments
Merge pull request #329 from hypothesis/222-update-hover

Annotator hover: update content/position when moving over a new highlight
Merge pull request #321 from waffleio/master

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Merge pull request #324 from okfn/custom-plugin-headers

Restore the ability for plugins to set Store headers
Make hover less sticky by following the cursor on changes, and simplify

Now we don't check for the privously displayed set of annotations,
just hide and show again.
Merge pull request #333 from hypothesis/222-hover-simplified-final

Make hover less sticky by following the cursor on changes, and simplify
pin watchify version for node 0.8 support

Wasn't sure if we care whether our build system breaks on node 0.8
at this point, but that seems to be the message in travis, which I
traced back to watchify introducing a dependency on chokidar.
Merge remote-tracking branch 'upstream/master'

Conflicts:
	src/plugin/auth.coffee
	test/spec/plugin/auth_spec.coffee