Remove MSB3270 (architecture mismatch) warnings when using fo-dicom NuGet package

b8cd906
The more important change in this pull request is a modification of the NuGet spec file / package to reference the native libraries using a custom .targets file. This will remove the obligatory warnings about mismatching target architecture on either the Dicom.Native or the Dicom.Native64 (or both) references when using the fo-dicom NuGet package.

13d4470
Second relevant change moves the common parts of all AssemblyInfo.cs and AssemblyInfo.cpp files to a single SharedAssemblyInfo.cs file, which makes it easier to maintain the version numbers, copyright etc.

The remaining commits 64c9211 and 436884b will remove some warnings from the build (arch mismatch for Dicom.Native64 and a missing reference to NLog), 8484214 adds the /ARM directory created by VS2012 and any generated NuGet-packages to the ignore list.