feat(build): use browserify for background scripts

Change all 4 content scripts and 1 background script (Opera):
- require app module, use app.name instead of <%= name %>
- remove ".template." suffix
- rename "inject" to "content" as they are content scripts

Enable browserify for these files, don't copy them in "meta" task.
refactor(background): require superagent with browserify

Use browserify to include superagent library in background scripts.
Add superagent to dependencies.
As this was the only bower dependency, remove bower-related stuff.
refactor(background): move absolute-url.js to source/meta/

- move absolute-url.js to source/meta/
- remove it from both firefox/chrome/content/ and opera/
- rename it to uri.js
- add CommonJS definition: export both functions inside it
- include it using require() via browserify

Yay! I wanted to do this so badly.
fix(build): don't copy .js files in "meta" task

Add filter "!**/*.js" which currently denies to copy
meta/opera/userscript-header.js
refactor(background): move handleAjax to handle-ajax.litcoffee

- extract handleAjax to source/meta/handle-ajax.litcoffee
- include it using require() via browserify
refactor(background): move inject to inject.litcoffee

- extract inject() to source/meta/inject.litcoffee
- include it using require() via browserify
docs(firefox background): note that Mozilla-fu taken from VkOpt
fix(dislike): in vk_dislike.users_info() use app.vkApi.request()
refactor(dislike): remove unused vk_dislike.auth() and .post()
fix(ajax): always use Content-Type "x-www-form-urlencoded"

Default one is "json" which vk.com doesn't understand.

This faces #83 (list of users disliked works correctly now).