Minor changes to stop code warnings appearing in Zend Studio 10.5.

Changes Made:
==========

- Changed double quotes around preg_match pattern to single quote in _preg_callback() function to stop "Bad escape sequence: \w" warning.
- Added "$start = 0" and "$taglen = 0" to beginning of _convert_blockquotes() function to stop "Undefined variable" warnings.

Notes:
-------
None of these changes cause the Htlm2Text class work in a different way to the previous version. Very much "cosmetic"!