Add Java installation instructions to INSTALL.OSX.rst

[ci skip]
Add a package.json for managing frontend dependencies

This allows us to install all but compass locally in the project rather
than relying on global executables.
Install frontend dependencies in bootstrap
Update webassets paths in configuration files

These now use project local dependencies rather than relying on them
being installed on the users PATH.
Update INSTALL docs to instruct on node installation

This reduces the number of installation steps for most platforms. Node
installation instructions taken from the Joyent wiki[1].

[1]: https://github.com/joyent/node/wiki/Installing-Node.js-via-package-manager
Merge pull request #1271 from hypothesis/node-modules

Manage frontend dependencies with npm
Merge pull request #1270 from hypothesis/osx-install-java

Add Java installation instructions to INSTALL.OSX.rst
Pin repoze.sendmail until upstream fixes known bug

repoze/repoze.sendmail#31
Add/Hack a script to autobuild Firefox extension

Like with the bookmarklet, the extension is created for the currently used
domain name. The building is only done the first time /hypothesis.xpi is
visited.

This code is very fragile and ugly, don't use it for anything serious!
Add annotator-store/reddit-proxy as requirement