#428 green: added ScriptExecution.HelloWorld

This is intended to be the Simplest Possible Thing™ for getting us started down the road of acceptance tests.

* The acceptance tests project does not reference *any* other project. It executes `scriptcs.exe` as a black box, mimicking a user.
* After execution, a `.csx` and a `.log` (console output) for each scenario is left behind in `test\ScriptCs.Tests.Acceptance\bin\{Configuration}\scenarios`. We can configure our CI server to collects these artifacts, e.g. http://teamcity.codebetter.com/viewLog.html?buildId=119644&tab=artifacts&buildTypeId=bt1253#!l0pcg0h53

#428