Removed all duplicated instances of getMangaMirror()

The js/lib.js file has been referenced instead for the
affected modules.
Renamed getMangaMirror to getMirrorScript in back.js

There's already another function with the same name.
Forgot some commas
Adding travis testing YAML file
Adding npm package.json
Removing node 0.6 version
Added badge to the readme
Added chrome.notifications test page

We may move from using Notifications in favor of the extension
API since the browser ones doesn't stack well.
Disables new mirrors notifications, for the moment
Replaced all extension.on/sendRequest for runtime.on/sendMessage

I hope this doesn't break anything
Added "return true" to onMessage method

According to http://stackoverflow.com/a/20077854/792066
we need to add return true if we would like the function to
be executed asyncronic, and that's what we would like.

Open new issues if you find other functions not being executed.
Refactored if statements in the onMessage listener

I changed the if change for a single switch..case.
The code looks more cleaner now and feels faster.
It needs extensive testing, through I checked out
the main functions something could have scaped my tests
specially, the bookmarks.
Removed old API
Bump version to 1.5.7
Merge branch 'release/1.5.7'