Relax rules that allow the parsing of a query string or an urlencoded body

The parsing of a query string or an urlencoded body was allowed, respectively,
for 'GET' or 'POST' requests. Now, this can be done for any kind of requests. Of
course, a body is parsed only if its Content-Type is set to
'x-www-form-urlencoded'.

In the same way, a body can now be parsed as a multipart content if its
Content-Type is set to 'multipart/form-data', regardless of the request type.

As a consequence of these changes, yaws_api:getvar/2 now returns the merged
result of values found in the query string and in the urlencoded body (if any).
Check #deflate.min_compress_size option for large files

Now large files are compressed only if their size is greater than
the #deflate.min_compress_size option.
Add support of wildcards and relative paths to load sub-config files.

Now, subconfig and subconfigdir targets accept relative paths, searched
relatively to the configuration location. So for the configuration
"/etc/yaws/yaws.conf", all relative files and directories will be searched from
"/etc/yaws". Of course, absolute paths are always supported.

furthermore, the subconfig target can take Unix-style wildcard strings to
include several files at once. See filelib:wildcard/1 for details.

Last, hidden files, starting by a dot, will be ignored in any cases.
Add the Travis build status in Readme.md
Remove support of wildcards for R15B03 and previous

A bug in filelib:wildcard/1,2 was fixed in R16B:
  https://github.com/erlang/otp/commit/70b5e24c

So, wildcard strings in the configuration file are now forbidden for older
releases.