Move make_unique to a subdir on include

This is to make it clearer that this is not a fundamental
part of the API, but rather just an auxiliary thing.
first inverted impl
Get the StatementWrapper to work, add tests
Put make_unique in the cxxdbc namespace

* Also fixed the make_unique include guard
Fixed formatting
Made Statement constructor explicit

* Also made it take an r-value reference instead of a value
Merge branch 'ruoso-master'