Added amount_abs to Entry model

This fixes issues with getting expense/income function to work. The calculation of direction is done on output, by the amount() method
	modified:   budget/models.py
	modified:   budget/templates/budget/budget_detail.html
	modified:   budget/templates/budget/transaction_list.html
	modified:   budget/views.py
	modified:   database.db
	modified:   filters/templatetags/filters.py