Added support for QR codes in Asset view

Use QR codes as asset tags, link back to Snipe-IT.

Print QR code onto sticker, slap on back of asset and :boom: no more typing in asset tags!

Requirements:
* Requires php-gd plugin.
* BaconQrCode installed through Composer.
* New database migration file.

## Asset View
![screen_shot_2014-01-28_at_12_22_18_am-2](https://f.cloud.github.com/assets/67991/2016488/862f8f80-87dc-11e3-8a86-9d7f4fdd083c.png)

## Admin Settings View
![screen_shot_2014-01-28_at_12_22_54_am-2](https://f.cloud.github.com/assets/67991/2016495/b0ecd836-87dc-11e3-8b40-99a7c7ca085a.png)

## Admin Settings Edit
![screen_shot_2014-01-28_at_12_23_01_am-2](https://f.cloud.github.com/assets/67991/2016497/ba436c9c-87dc-11e3-9096-08f017868184.png)