Merge pull request #25 from elitenomad/federico

Federico
Added OWE or OWED concept
Merge pull request #27 from elitenomad/Pranava_Users

Added OWE or OWED concept
add aasm gem to manage status
add column settled to expenses and portions
added status for groups
added settled for expenses and portions

default values for groups is open, for expenses and portions is false
modified expense , portion and group model

add scope to expense and portion