Update Html2Text.php

Small additions/changes to stop code warnings appearing in Zend Studio 10.5.

- Changed double quotes around preg_match to single quote in _preg_callback() function to stop "Bad escape sequence: \w" warning.
- Added $start/$taglen = 0 to beginning of _convert_blockquotes() function to stop "Undefined variable" warnings.
Update Html2Text.php

Code clean up.
Merge pull request #10 from alanspencer/master

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