Create abstract base class for Advimport source helpers

The MsAccess and PHPExcel helpers share quite a bit of common code, and I thought it made sense to build an abstract class for them.

The motivation is that PHPExcel is giving me trouble. There's the memory issue for large imports, which we've previously discussed. It also seems to be dropping the first column of some of my CSVs. I decided to make a CSV helper, which I'll submit when it's done, but I thought this code improvement should come in separately.