Independent values for img alt attributes

Creating a new **Alt:** field in the slides accordion where the alt attribute of each image can be defined independently of the visible slide title text. Includes support for alt attributes in all applicable default themes. Original question over at #67 

![image](https://cloud.githubusercontent.com/assets/1401894/4377132/d30721e4-434b-11e4-80ad-0a822bef3db4.png)