Remove dead code
Support document access strategy failover

Detect when scanning with the selected document access strategy fails,
and try to fall back to other strategies.

(This will allow using the Dummy strategy when d-t-m fails for some reason.)

Also, fix TextPosition strategy to avoid crashing without d-t-m.
Simplify code
Simply drop the failed strategies

... instead of marking them as failed.
Introduced separate _removeCurrentAccessPolicy() method
Use simple JS instead of Coffee magic
Merge pull request #79 from hypothesis/strategy-failover

Support document access strategy failover.
Merge remote-tracking branch 'hypothesis/typed-packaging' into typed-packaging-MSA-2
ck--/ann#470 do not crash when d-t-m cannot keep up