Fix EvalScript by using verbatim string for #Load

This is in reference to fsharp/FAKE#501. This fix passes paths verbatim
so that paths such as C:\code\build.fsx do not fail due to the escape
sequence '\b' being present.
Add regression test for #184
Merge pull request #184 from petersondrew/master

Fix EvalScript by using verbatim string for #Load