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
add failing test for package root spec static view
Fix static views with package root spec patterns
Add myself to contributors
Format proutes output and include module instead of repr of view
adjust tests to work even when someone has defined PYTHONSTARTUP in their shell