move database converter into its own file
add utility for generating queries based on views

will be used to combine user directory entries with a search
convert UserDirectoryView model into a namedtuple

Since it's a view, data doesn't need to be modified.
In that case, it's safer to use a namedtuple for storing
data.
add mobile phone number and agent id to user helper
add view for user directory entries
remove old view directory code
Merge pull request #21 from xivo-pbx/refactor_user_view_2

Refactor user view 2