add "rebuild" param to utils' script, to force dockers rebuilding

it makes more sense to rebuild dockers all the time on stable branches,
because we don't have to count on DockerHub's policy to keep our
docker images on their servers.

docker: make build- and push-image scripts more verbose

Merge stable-1.5 into stable-1.6

otherwise it will still use found (most likely, newer) clang-format.

Update doxygen settings

bump docker images versions

and set clang-format to fixed version (6.0).

fixes #854

Merge stable 1.5 into stable 1.6

contigous_iterator: make operator[] take signed integral instead

of unsigned.

The reason is that Windows cannot decide for a call like this:

`it[0] = 10`

whether to use:
* operator[](unsigned) -- one conversion from signed to unsigned
* operator T*() and then build in operator[](signed) -- one conversion

Merge stable-1.5 into stable-1.6

fix dir creation in auto docs update

doc: enable auto gh-pages generation in docker script

Create separate Docker images for stable-1.6 branch

tests: remove unnecessary #include <libpmemobj.h>

Merge changes from stable-1.5

allocator: remove unnecessary #include <libpmemobj.h>

Including libpmemobj.h causes inclusion of all type macros
which are not needed by C++.

Including libpmemobj.h causes inclusion of all type macros
which are not needed by C++.

Merge back changes from stable-1.5 to stable-1.6

[1.5] Do not check pkgconfig on windows