Also update the list of headers cowboy_rest might set
and tweak a small number of other items.

It is now tested both via cowboy_req:read_body and
via cowboy_req:cast.

Removes a bad example from the guide of body reading
with period of infinity, which does not work.

The read buffer was changed into an iovec to avoid doing
too many binary concatenations and allocations.

Decompression happens transparently: when decoding gzip,
the content-encoding header is removed (we only decode
when "gzip" is the only encoding so nothing remains).

We always add a content_decoded key to the Req object.
This key contains a list of codings that were decoded,
in the reverse order in which they were. Currently it
can only be empty or contain <<"gzip">> but future
improvements or user handlers may see it contain more
values.

The option to disable decompression was renamed to
decompress_enabled and defaults to true.

It is no longer possible to enable/disable decompression
in the middle of reading the body: this ensures that the
data we pass forward is always valid.

Various smaller improvements were made to the code,
tests and manual pages.

LH: I have added a test that does both hibernate and timeout
    and fixed a related issue. I also tweaked the docs and tests.