Replaced lexer with a new version based on a depth-first left-to-right traversal of the AST. The new lexer allows styling nested Markdown based on the innermost AST node.
Added unit tests for MarkdownTokenListBuilder.
Refactored MarkdownTokenListBuilder to simplify testing.
Switched matcher base class to TypeSafeMatcher.
Added high-level unit tests for the lexer.
Removed unused token type TEXT.
Added unit tests for the MarkdownLexerVisitor when used with the PegDown extensions over standard Markdown.
Changed default token type to PLAIN.
Added some Javadoc.
Merge pull request #33 from stengerh/lexer-dfs-ltr

New lexer for syntax-highlighting