increment version to 1.1.2

Most significant fixups involved supporting move-semantics for

arguments to gettext_str and friends. When possible we will
avoid copying the input string now.

Besides this we test and support msvc 2013, and removed some
unnecessary constexpr declarations.

We also moved the "main" header, so that the include path is
`<spirit_po/spirit_po.hpp>`, which seems more sensible than
the previous path.

There were some other mostly superficial changes. Forgotten
typedefs, warning suppression on certian compilers, etc.

Fixes small problems in the source

First stable relesae