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

fix dir creation in auto docs update

doc: enable auto gh-pages generation in docker script

allocator: remove unnecessary #include <libpmemobj.h>

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

[1.5] Do not check pkgconfig on windows

it might be installed (as part of cygwin).

Backport appveyor update to stable-1.5

... to reduce the Appveyor build time.

... to reduce the Appveyor build time.

PMEM_IS_PMEM_FORCE is not set in tests
and Appveyor build is timed out.

This change is required because of the following issue:
https://help.appveyor.com/discussions/problems/24684-problem-with-vcpkg-wrong-version-on-appveyor
https://github.com/appveyor/ci/issues/3052
which cannot be fixed directly now.

Extend warning message when valgrind is not found.

Install libpmemobj++.pc in all configurations

Specify we need devel package.

Adjust TRAVIS_COMMIT_RANGE variable to contain only commits from a PR.

to contain only commits from a PR.

Ref: travis-ci/travis-ci/issues/4596

[1.5] cmake: check if dot tool is installed

which is required by doxygen to generate graphs

[1.5] fix typo

Fix crash when previous transaction failed to start

... because of already taken lock.

Bump pmdk to 1.5.1 in Docker images.

Throw an exception when unlocking failed.

backport: Disable some libcxx tests for old g++ compilers.

They cannot be compiled due to lack of full C++11 support.

stable1.5: fix persistent ptr

This reverts commit 6bbb319e.