Refactor MbRegex::prepareReplaceArgs to be more readable and fix callback issue

- add tests to cover array callback to MbRegex and PcreRegex
Fix moved Bench directory in README
Use a bit of GFM to make README more readable
Use more self explanatory method names for setting up an error handler and removing it

- change prepare to setUp
- change cleanup to tearDown
Refactor README
Add MbBench and extract StringBench from PcreBench
Implement MbRegex::split with native functions to support options
Update README with new benchmarks