Explicitly set Invoke Return Variable

[McOso]

Related to #7

Problem: Compile warning

warning[5667]: Warning: Unused function parameter. Remove or comment out the variable name to silence this warning.
   --> lib/delegatable-sol/contracts/Delegatable.sol:128:18:
    |
128 |         returns (bool success)
    |                  ^^^^^^^^^^^^

Solution: Explicitly set the success return variable of all external invoke functions.

Change increases gas consumption for the invoke function by 3

[danfinlay]

Looks fine to me!