[FIX] res_currency: avoid getting blocked when creating a curreny without rates
[IMP] res_currency: more modular solution
[FIX] res_currency: update linked tests
[FIX] res_currency: pass context to currency function
[FIX] res_currency: allow silent failing for computation of current rate if no rate exists, allow accessing the tree and form view if incorrectly configured currency
[MERGE] Forward-port of 7.0 bugfixes up to rev. 5041 (mat@openerp.com-20130731145128-flzp1imkyrh86zpa)