Fixing issues with freeing regex.
pcre_free() used instead of ordinary free().
Redmine#2038: adding string_split function to replace splitstring

Deprecate splitstring()

Add StringMatchWithPrecompiledRegex() and CompileRegex() to avoid
compiling regex multiple times.

Fix compilation warning about const Rlist.
Merge pull request #1378 from tzz/tzz/pasinskim/feature/split_string

Redmine#2038: add string_split function