Add custom `boost::locale::numpunct`

[Flamefire]

Rebase of #65 onto develop with current changes incorporated. Hence closes #65

Contains the original commit(s) from @salvoilmiosi of #65 squashed and rebased onto develop to fix the conflicts.
On top some refactoring and documentation additions to suit current state of the library.

Will also contain tests.

Design:

• provide boost::locale::numpunct for all backends
• add decimal_point_str & thousands_sep_str to provide possibly encoded variants of the decimal and thousand separators
• decimal_point & thousands_sep return best-effort single-chars, by default falling back to the "C" locale
• Tests should ensure that a Boost.Locale agnostic library (i.e. reading facet.thousands_sep() & facet.decimal_point() and doing "regular" streaming using std::locale) get the expected results.