Memory and Speed optimizations

Features:

• Added --debug option to show a memory usage increase from init to final run

Patches:

• Release memory by unloading assets with no outgoing relations. Fixes #117
• Switch external link checks to HEADrequests to reduce network load
• Fall back to GET requests for external requests when encountering known errors with HEADrequests
• Send better Accept-headers to reduce errors from external servers