Require "payable" modifier if function wants to receive ether.

[chriseth]

Work in progress

Replaces #665

Partially implements #500 and #563. Comments welcome.

Fixes #602.

todo:

• library function should not be payable
• contract is payable should create payable fallback function (and error if there already is one)
• calling a non-payable function with value set should report an error
• accessor functions should not be able to receive ether (-> callvalue check should probably move to dispatcher)
• internal functions should not allow payable modifier
• overwriting a base contract function should not change payable