add support for using an absolute path to override an asset

fixes #1229
Merge branch 'master' into feature.override-asset-with-absolute-path
Docs: Introduce the concept of "userid" into the glossary.
Docs: Make clear that a userid need not be a principal.
Security: Change "principal" argument in security.remember() to "userid".

Make the change througout the authentication policies, etc. as well.
Docs: Make "userid"  link to the glossary term.
Docs: Include the concept of credentials in the high level security overview.
Docs: Add resource tree into security overview.
Docs: Switched first 2 paragraphs of security overview.
some tweaks to the usage of userid in the docs
Merge branch 'master' into feature.security-docs-enhancements
re-add missing Interface parent
update the public api for remember
update changelog
enable PYTHONSTARTUP support in pshell

Fixes #1299
adjust tests to work even when someone has defined PYTHONSTARTUP in their shell
attempt to clarify the benefit of these changes
improve the docs for absolute path overrides
reference appropriate PR
Merge branch 'master' into feature.security-docs-enhancements