Fix duplicate '100-continue' bug.

Node.js automatically responds with a 100-continue if the 'checkContinue' event isn't handled (http://nodejs.org/api/http.html#http_event_checkcontinue).

We tried to use citeproc-node via web requests made in C#, but were unable to deserialize the json results.  Through some investigation, we found that our POST requests had their Expect headers set to '100-continue' by default.  Since node.js and citeproc-node sent back 100-continue responses to this, the deserialization of the call ultimately failed.

Repro steps:
- Start an instance of citeServer.js locally on port 8085.
- Run the following command via curl (http://curl.haxx.se/):

        curl -H "Content-type: application/json" -H "Expect: 100-continue" -X POST -d "{\"items\":[{\"type\":\"book\",\"author\":[{\"family\":\"Lichtheim\",\"given\":\"Miriam\"}],\"title\":\"Ancient Egyptian literature: volume II: the new kingdom\",\"issued\":{\"date-parts\":[[\"1973\"],[\"0\"]]},\"publisher-place\":\"Berkeley\",\"publisher\":\"University of California Press\"}]}" http://localhost:8085?style=apa

- Expected: citeServer returns a bibliography object with the desired results.
- Actual: the response from citeServer fails to return.


- Also made a change to return a 400 when the requested citation style isn't in the /csl folder.

- Another change adds a "forceLang" query parameter that allows a client to specify that, even if a csl format has a "default language" (i.e. din-1505-2), the language specified in the query should be used.

- Another change makes sure to specify the charset as utf-8 in all of the Content-Type headers.