Add [Pure] to fluent BDDfy methods

[JakeGinnivan]

By doing so R# will warn when you do not add .BDDfy() at the end

[mwhelan]

Sounds useful. What's Pure?

[JakeGinnivan]

A hint to r# that the method does nothing. An example is myString.Replace("foo", "bar") will warn because Replace is Pure (you need to assign output for it to do anything).

Given/When/Then are all pure, only .BDDfy() does anything

[SonOfSardaar]

fixed by #298