improved the loading of content scripts

Instead of always loading jquery, both its plugins, and back.js (a
fairly large file with a lot of functions) on every page we can instead
just load the 9 line file testPage.js which will send a request to the
background page to inject the necessary content scripts along with the
implementation script.
Non-manga pages can be identified in ~ 40ms without loading any
unnecessary files (from my testing).
move definition of contentScripts to the top

Decided to move the scripts array to the top background.js and rename it
to contentScripts in order to be more clear and to possible allow for
dynamic adding of content scripts. For example a setting in the options
page could trigger background.js to push another script into the array
extending functionality but not forcing the script to be always loaded
unless indicated by the user.
Fixed download button

It downloaded the complete html file, and triggered DOM Exeption 5

@fuzetzu
fix mime type

@braiam
I was just looking it over and I made a mistake in my original posting
(in the forums), the proper mime type is text/plain, text by itself
works because we want to download it anyway but it is an invalid mime
type.
Merge pull request #86 from AllMangasReader-dev/feature/less_content_scripts

improved the loading of content scripts