[#120] Allow donations to fully fund projects

Add Project.amount_needed_to_fully_fund_via_paypal() and
Project.amount_needed_to_fully_fund_via_ideal() that calculate how
large a donation is needed to fully fund the project when transaction
fees are included

Add settings for the transaction fee calculations

Add text to donation form page that tells the user how much is needed
to fully fund the project including projected transaction fees using
the methods above

Change the validation to allow for the "overfunding" above

Fixes related to the switch of memcached library