Clang tidy/auto ptr

[filodej]

Hi, Fist I would like to thank you for a great piece of software and also for making it more available thanks to the recent licence changes.

I am trying to modernize CppCMS (primarily get rid of the deprecated std::auto_ptr). I have used clang-tidy 5.0 with the modernize-replace-auto-ptr check, so the first commit is simply output from the tool. The second commit contains my manual changes in order to fix the build (with gcc 7 in c++14 mode) and subsequently fix all tests.

So this pull request contains mainly the following changes:

• std::auto_ptr has been replaced with std::unique_ptr
• booster::shared_ptr and booster::weak_ptr (and few related classes and functions) have been replaced with aliases to std equivalents (could be solved differently - e.g. by changing the CppCMS code to use the std classes directly).

I understand that this is the API change and so it probably could be included only to a major version. I just wanted to share my effort and hear your opinion (I am probably going to make more changes/modernizations based on other clang-tidy suggestions).