added gzip/deflate support

Allow body-parser to accept gzip/deflate-compressed bodies (in POSTs, e.g.).

Worth discussing whether my handling of length and encoding is correct.

Finally -- no test, as the current supertest does not support posting arbitrary buffers and/or streams. We can either create our own request for this specific test, or modify supertest, or skip the test, or use another request library (requests?).