Removed unneeded references
Fixed few warnings
Changed the default to not use RegexOptions.Compiled

RegexOptions.Compiled for some reason resulted in insane execution times
(scan_autolink_uri taking 10 seconds and huge memory, for example - not
startup/compilation time but execution/match time). This probably needs
more testing to determine if the same applies to all of the expressions.
Merge branch 'master' of https://github.com/Knagis/CommonMark.NET