Added /login to users.py; see #128.
Fixed #128. Implemented last login and /login in users.py
Fixed #132. Guard POST /groups with validation.

We require 'name' to be present and it cannot be an empty string. We also ensure that if users and sites are provided, they must exist.
Merge branch 'issue128' into issue132
In addition to #132, we fixed #136 to send update to groups in minion-db-init.
Fixed #133. Remove user's associations in groups and sites when deleting invites.
Fixed #135. GET /plans will also include 'created' as part of response.
Merge pull request #129 from yeukhon/issue128

Issue128 add /login and last login
Merge pull request #131 from yeukhon/master

Fixed #130 include minion.backend.views in the packages
Merge pull request #137 from yeukhon/issue132

Issue132 Guard POST /groups with validation
Merge pull request #138 from yeukhon/issue135

Issue135 include 'created' field in GET /plans
Merge pull request #139 from yeukhon/issue133

Issue133 Remove user's associations when deleting invites
Close #144 for now. Allow tests to pass by not importing zap/nmap in tests.

Since 8091f3c7 we can't import plugins that are invalid (namespace not exist for example).