move command execution code out of builder class
fix issue #218 by blanking last output before executing new command.
add tests for CommandExecutor.
add return value tests for CommandExecutor.
move findBinary from Builder to CommandExecutor.
rename CommandExecutor::executeCommand() so that it's not confused with the Builder::executeCommand()
Merge pull request #238 from meadsteve/command-execution

Move shell command execution out of builder into separate class and fix issue #218