Fix bug which lead to infinite loop when parsing strings containing $0 (e.g. http://commons.wikimedia.org/wiki/File:Yangzhou_-_supermarket_-_watermelons_-_P1070029.JPG which has

description = "{{en|Watermelons of 3 varieties sold in a Yangzhou supermarket. This is January, so they must have been brought from somewhere like Hainan, and are fairly expensive (4Y, i.e. $0.70, per pound).}}" )

By using str_replace whenever a constant string is being replaced, rather than preg_replace, we avoid using regexp substitution in the replacement string, and the code is also simpler and faster.
Allow strip_tags to strip nested tags such as <b><i>foo</i></b> or even (as occurs in parsed wikitext) <b><i>foo</b></i>. Also strip <br> and <br /> tags (which do not have closing tags).
Merge pull request #64 from hyanwong/wikiparser_preg_bugfix

Wikiparser preg bugfix