We thought that this kind of rewrite can be achieved by the configuration
of the backend severs, but in some configuration, however, it may get
complicated. So we decided to implement at least location rewrite in
nghttpx.

This commit also contains a fix to the bug which prevents the http2
backend request from concatenating header fields with the same value.

It may violate draft-09 section 8.1.3.1. Proxy should perseve
host header field.

To build extension module, cython is required.
The module is not built with `make` in the top directory.
A user has to run `make build_ext` in python directory.
Currently header compression objects are available for testing.

We tried several times about this subject, but for the current
HTTP/2.0 priority scheme, we think it is best to serve the highest
priroty streams first (interleaving streams if there are several
higest ones). There are an issue when aggregating several frontend
connections to one connection in backend, but it is HTTP/2.0
spec issue, rather than implementation.

For now we just double the priority value on each DATA frame
transmission. If priority is 0, it becomes to 1 and goes all
the way to (1 << 30) - 1.

This is pathological case, but we may get RST_STREAM to the promised
stream while we are sending PUSH_PROMISE. To handle this case,
instantiate Request object before transmission.