Create LocalProcess helper

[danepowell]

#713 resulted in a drop in code coverage because we added new methods to LocalMachineHelper that had to be mocked and therefore aren't tested. The problem is that LocalMachineHelper as a whole was already being mocked and there's no way to call just some of the original methods with Prophecy (phpspec/prophecy#332 (comment)).

Per the Prophecy maintainer, this is a sign that LocalMachineHelper violates SRP and I'm inclined to agree.

I propose refactoring LocalMachineHelper to move execute() and executeCommand() into a dedicated LocalProcess class. This could be mocked on its own and neatly encapsulates and isolates interaction with the local shell, which is 90% of what's being mocked today.

We could even extend this class later on to create helpers for specific oft-used processes like Git.