Small exec.js cleanup, clarify port options settings in README

[cgewecke]

Resolves #42.

Kind of copped out here but basically - if you're passing in a testrpc options string, you should specify a port there, & also set the .solcover.js port option, and your coverage network port should not conflict with these settings. Otherwise tool will break.

[codecov-io]

Codecov Report

Merging #47 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #47   +/-   ##
=======================================
  Coverage   97.56%   97.56%           
=======================================
  Files           6        6           
  Lines         329      329           
  Branches       78       78           
=======================================
  Hits          321      321           
  Misses          8        8

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[cgewecke]

Resolves #42