Ability to have PerfSetup method per test instead of per class

[wsaeed]

Currently NBench only supports PerfSetup method per class. Normally a test class contains multiple test cases. Having a single setup method for all tests slows down the test execution as setup has to be done for each test, while used only for one. It will be ideal to have support for specifying a PerfSetup method for each test.

Suggestion:
Add another property in PerfBenchmark Attribute say PerfSetup Method and execute it if specified.