Fix a bug when watcher email is nil.
add database and models for groups
add groups today report api
modify group today api
add api to get/post group users
change home page to show groups
add 'add_group' page
add group settings page
add groups settings link
change home page design, integrate groups buttons

- move 'Today' link to a button
- add dropdown list for 'report list', 'add group'
not use img for user name
rename db migrate script after rebase
change home page after rebase
add groups page
add join/leave group implementation
add no group page, redirect user to it when they belong to no group
remove bootstrap.js while we use angular-bootstrap