add initial user index implementation

just a list of users sorted by reputation
force size of gravatar in post user summary
add question form styles to question#edit
add gravatar to list
Merge branch 'master' into user-index
add reputation & pagination to list
Change UserController to use require_user for edit authorisation
update gems
fix issue where long usernames break page layout
increase spacing between elements
Merge branch 'user-index'
add link to user index in header
update gems
Merge branch 'master' into autocomplete-tags