kick off fetches for external resources in parallel

For the parallel enhancement, my test .scxml contained a datamodel like so:

  <data id="d0" src="sleep-json.cgi?secs=5"/>
  <data id="d1" src="data1.json"/>
  <data id="d2" src="data2.json"/>

Here's the log:

2013-01-14 22:56:10.749 FLDR: INFO HTTP GET BEGIN [uri=http://localhost:8888/scxmltests/misc/sleep-json.cgi?secs=5]
2013-01-14 22:56:10.751 FLDR: INFO HTTP GET BEGIN [uri=http://localhost:8888/scxmltests/misc/data1.json]
2013-01-14 22:56:10.754 FLDR: INFO HTTP GET BEGIN [uri=http://localhost:8888/scxmltests/misc/data2.json]
...
2013-01-14 22:56:10.765 FLDR: INFO HTTP GET END. [uri=http://localhost:8888/scxmltests/misc/data1.json]
2013-01-14 22:56:10.765 FLDR: INFO HTTP GET END. [uri=http://localhost:8888/scxmltests/misc/data2.json]
2013-01-14 22:56:15.951 FLDR: INFO HTTP GET END. [uri=http://localhost:8888/scxmltests/misc/sleep-json.cgi?secs=5]

Also added mimeType parameter to getResourceFromUrl API to suggest a mechanism (via the HTTP Content-Type) for the interpreter to determine how to process/reflect the resulting content. In hindsight, I should have saved that for a subsequent issue/discussion.