Added IL code weaver and AllureAttachmentAttribute to support an attachment mechanism.

This pull request is regarding issue #3 in the allure-csharp-commons.

The technique I chose was to use mono cecil to inject IL code into methods marked with a special attribute.
The injected code fires an event with the method's return value that a is passed to anyone registered to the event and where it can be recorded.

The test container is altered during this process and a new test container is created on disk.

The reason for the pull request is not necessarily to merge it as is but to discuss the method used for the implementation.