Simplified AddStep function

1. Right now, AddStep doesn't return errors. Errors are logged in Run() function instead.

Before:

err := suite.AddStep("", someFunc) // err contains an error
if err != nil {
    t.Fatal(err)
}

after:

suite.AddStep()
suite.Run() // test fails here

It should simplify the end-user code.

2. AddRegexStep function added #56