Added bullet character to those that mark a list item

This is not in the specification but a very reasonable thing to do.
scan_atx_header_start rewritten without Regex
Small optimizations
scan_setext_header_line rewritten without Regex
Small perf improvement by reducing get_Next() invokes.
HtmlTextWriter optimized by implementing Write(string)
scan_entity rewritten without Regex
scan_link_url rewritten without Regex
scan_link_title rewritten without Regex
Removed remaining regular expressions
Small perf improvements
Updated version number and performance data.