More consistent source values

Changes

• Previously, when using source('/some-path') you would always get a store with the first value initialized to blank.
  
  That doesn't make much sense if you're connecting to a source that had already open and had already produced some data.
  In that case, the returned store used to emit:
  1. First an undefined value
  2. And then the most recent value of the stream
     
     This change makes it so that the returned store emits directly the most recent value of the stream if available.
     For more details see #48