Mainly fixed and completed the AT grammar.

- updated the AT parser documentation
- fixed a little the test input for the parser

AT Grammar:

- block nodes are now flagged as "block"
- changed term "element" to "node": more generic, and consistent with HTML
- some unnecessary code/rules have been removed
- changed some precedence in rules for better accuracy and consistency too
- completed the terminals encapsulation in nodes for many rules
- fixed CDATA content parsing (made it optional)
- finished implementing the parsing of expressions (with modifiers)
- fixed some little quirks (like allowing spaces at wrong places, setting properties with nodes, ...)