make PCRE++ pcre4 ready:
 - add PCRE_ERROR_MATCHLIMIT support to the exception class
 - add another constructor with match_limit support (pcre_extra data to pcre_exec)
 - add a new method define a callout function which maybe called by pcre in
   case (?C) occures in the regex. See change #28 and #49 in the pcre changelog
 - add a new method to retrieve named submatches
 - add a wrapper for pcre_fullinfo()

- add a method to enable pcre_study().
- add operator[] for directly matching substrings
- add method to change flags after the constructor has been called
