yara: add dotnet module

yara: add dotnet module

Dotnet module added as a compile time option.
The dotnet module allows you to create more fine-grained rules
for .NET files by using attributes and features of the .NET file format.
http://yara.readthedocs.io/en/latest/modules/dotnet.html

Signed-off-by: Ashkan Jazayeri <ashkan@jazayeri.net>