Allowing user to add leaves which may results in negative leave balance.
Merge branch 'master' of https://github.com/webnotes/erpnext
Added checks to see if the selected leave type allows negative balance. If it does then warn the user and continue with saving the form else just warn the user and don't save the form data.
Added a new field called allow_negative. This field was added for allowing a negative balance for a particular leave type.
changed the logic to be more elegant.
Merge pull request #300 from mayur-patel/master

Allow user to add leave application which may results in negative balance.