Wrap EM around tagged examples only

Hey,

The current implementation runs every single rspec example within the context of an EM reactor loop. For an EM-unaware, synchronous code this is an unnecessary performance overhead. 

To address it, I added an "eventmachine" tag which decides if given example is to be wrapped in an EM loop, e.g.:

    describe Foo, eventmachine: true do
    ...
    end