Remove register storage specifier

Remove register storage specifier to silence clang warnings "'register' storage class specifier is deprecated".
Merge pull request #3 from Lastique/patch-1

Remove register storage specifier
Remove use of deprecated register keyword, see: https://svn.boost.org/trac/boost/ticket/9640
Merge branch 'develop' of https://github.com/boostorg/regex into develop
Doc update.
Merge branch 'develop'