fixing issue in http-duplex

- the test for node 0.8 did not work therefore core streams were used at
  all times
- readable-stream wasn't installed, so even if test would have worked it
  would have failed when trying to require it
- now using readable-stream no matter what the node version is, in order
  to be consistent across all versions
- making it a dependency as well (anything that gets required, even
  conditonally, should be)
Merge pull request #18 from thlorenz/fix-http-duplex

:+1: much better. Merged.