fix(background): Tweak logic to inject script programmatically

[ERosendo]

This PR fixes freelawproject/recap#346.

The current implementation of the extension inserts scripts listed in the CONTENT_SCRIPT_FILES array calling the executeScripts method sequentially inside a for-loop. I think this approach is causing the bug in Firefox because the executeScript is an asynchronous function, so there may be calls that are still running when the next one starts (which is not desired).

We need to make sure the extension injects a new script after the previous one succeeds and so on. Luckily we can pass a callback functionexecuteScripts method so we can nest the executeScript calls like this:

chrome.tabs.executeScript(null, { file: "jquery.js" }, function() {
      chrome.tabs.executeScript(null, { file: "master.js" }, function() {
          chrome.tabs.executeScript(null, { file: "helper.js" }, function() {
              chrome.tabs.executeScript(null, { file: "client.js" })
          })
      })
  })

The previous code makes sure the master.js script is injected after the jquery.js file and so on, but the nesting can get unwieldy so this PR adds a helper function to abstract the previous approach and handle the programmatically script injection.

[mlissner]

One suggestion about comments, but otherwise looks good to me.

It's all good