Add config setting for asset path

[mattpass]

ICEcoder's asset files (CSS, JS and images) might be held in another location (eg a CDN, S3 etc) to ICEcoder and a config setting should be added to define and also use this.

A 2nd cache busting setting may also be added to allow cache busting or not.

These 2 values should be set in the global config file in data dir.