Fix crash on new purchase order when no spendable bitcoin outputs exist.

Fix crash on new purchase order when no spendable bitcoin outputs exist. Error:
/var/lib/gems/1.9.1/gems/mastercoin-wallet-0.0.11/lib/mastercoin-wallet/builder.rb:114:in `pick_outputs': undefined method `reject!' for nil:NilClass (NoMethodError)
Merge pull request #7 from Exocyst/patch-1

Fix crash on new purchase order when no spendable bitcoin outputs exist.