Add OpenCover and ReportGenerator tools

According to OpenCover - the JSON.NET tests (.NET45) cover ~90% of sequence points and ~80 of branch points

Visited Classes 208 of 226 (92.04)
Visited Methods 2146 of 2403 (89.31)
Visited Points 10991 of 12335 (89.10)
Visited Branches 8733 of 10884 (80.24)

Also added reportgenerator to help visualize the results