v0.16.0

Changelog:

5accaad Require download policy to be stricter
109f51e Verify signatures of indexes when loading from disk
cded243 Download missing sigs
44dc8df Save verification options to resource and save if versions have a sig available
77a6ab0 Add path existence check to utils
beaa748 Add support for signed indexes
f6fc67a Add support for new index format
0e5eb4b Add support for signed updates