Enabled theme to be configured in book config; fixed AV embed urls when av_root_dir is set

This commit adds book configuration options ```theme``` and ```theme_dir``` which, as the names suggest, make it possible to change the theme directory and the theme name through the book configuration. By default, the values that were used previously are used (haiku and RST/source/_themes).
In addition this fixes the paths to AVs/Exercises if the av_root_dir is specified in the configuration.

Since I'm no expert on the configuration process I didn't want to directly commit this to the repository. Hopefully someone can look through this and merge if it's okay. I'm happy to make changes if required.