Merge pull request #69 from rudimeier/travis

Travis continuous integration
Fix #59. It now checks at post and if content length is zero, it just processes the request.
Update README.rst
Update README.rst
Fixed coverity and travis-ci badges.
Badges are now proper links.
Fixed coverity found bugs at main library (src/onion).
Merge branch 'master' of github.com:davidmoreno/onion
Fixed coverity found bugs at tools. There is, maybe, a remaining memory leak.
Fixed coverity found bugs at examples.
Fixed coverity found bugs at tests.
Fixed bug at updateasset that close not exisiting nor open asset.h file. Fixed return type on 06-onion test.
build: allow user to override CFLAGS

Now something like this works as expected:
 cmake -DCMAKE_C_FLAGS="-O3 -Wno-error"
build: remove -Werror from default CFLAGS

It usually break on each untested compiler version. That's annoying
for the user. On our own devel machines we can always explicitly set it
and we still set it per default in debug mode.
build: set CXXFLAGS also like CFLAGS
travis-ci: compile again with -Werror

-Werror was removed lately from default CFLAGS. In CXXFLAGS it was
never used until now.