Add GROUP_BY_* constants and update WITH_OFFSET.

In this way, we have aliases for all PCRE constants used by split() and
match(). WITH_OFFSET works for both.
[auto] Merge from String

    • #9bc3761 — Add GROUP_BY_* constants and update WITH_OFFSET. (Ivan Enderlin, 2013-08-16)

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Merge remote branch 'library-string/master'