Add SlotAsset get api, with possible format

No options (for now?)
Remove unnecessary SlotAsset.containingSlot
Use SiteExceptions for action handling
Add slot arguments to asset update/create for linking
Relocate api-spec.txt from design
Make asset.name optional and remove body
Add revisions option to asset.get
Use relative paths in ingest.asset
Content for NotFound and Forbidden errors

Fixes assembla-159
Merge remote-tracking branch 'remotes/origin/master' into angular
moved api docs to routes
updated js to follow api