Fix to Excel related project configurations so as to reference included Office Interop dependencies

Included local references to Microsoft.Office.Interop.Excel.dll and Office.dll by copying them to new "../extern/Office" directory and setting "Copy local" to true to copy them over to output directory. This way all users having an Excel installation will be able to run Excel nodes by referencing the same version of these libraries. Currently they should work with Excel 2010 (both 32 bit & 64 bit) installed on user machines. Yet to test with higher versions of Excel.