Refactor options / exec.js cleanup

[cgewecke]

Options are pretty much incoherent ATM - esp. if someone uses the testrpc options one. Gaslimit should be hardcoded into testrpc-sc if it isn't already.

There's also a problem with the port . . . for example someone who doesn't specify a port but DOES use testrpcOptions will automatically run on 8545.

Also replace all instances of ${coverageDir} which are not part of a larger string. Should be coverageDir.