Expression uri

Fixes #59.  I'll leave this pull request open for at least 24 hrs before merging to allow for comments.  

With this change, the InetSource operator determines (using hasSideEffects) whether the URL expression should be recomputed before every fetch.  If not, nothing changes.  If so, then before every fetch, all URLs are computed.  When the URL has changed, the last-modified time is reset.  

I added a sample, InetSourceDynamicURL with a somewhat strained example of this feature.  

Also, I had to make a few modifications to the InetSource to support this.  InetSource was using a map (A->B), and I changed that to use two vectors, a vector of A's and a vector of B's, with the understanding that the ith A corresponds to the ith B.