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
Merge branch 'master' into feature.security-docs-enhancements
note the deprecation in the todo
Merge branch 'master' into feature.security-docs-enhancements
Merge branch 'master' into feature.security-docs-enhancements
Merge pull request #1399 from Pylons/feature.security-docs-enhancements

security docs enhancements