Honour pre-processor if/unless statements (fixes #413)

Introduce pre-processed if directives to hide code from Opal. Two special
constant checks now take place in the compiler. Either `RUBY_ENGINE` or
`RUBY_PLATFORM` when `== "opal"`. Both if and unless statements can pick
up these logic checks:

    if RUBY_ENGINE == "opal"
      # this code compiles
    else
      # this code never compiles
    end

Unless:

    unless RUBY_ENGINE == "opal"
      # this code never compiles
    end

This is particularly useful for avoiding `require()` statements being
picked up, which are included at compile time.