Begin Pruning Bottom-up Implementation

- Some util.js name refactoring. Renaming util.js
  -> utils.js. Moved regular expression-based
  functions into utils.js.

- Added some type checking, string replacement
  and checking functions to utils.js

- Blank string guards in image alt attributes,
  link display text, and ATX header text. If the
  text in these elements are just blanks (Spaces
  and line breaks), these elements will not
  appear in the AST.

- Removed ASTNode.prototype.appendChildren(),
  as the project no longer uses it.