Markup spec/ANTLR/Images

The following code does a pretty good job of handling images. Obviously all the actual inline text processing and definitions of text are primitive.

Notes:
 * It can handle nested images
 * It can handle links in captions, which don't trouble it at all: [[image:foo.jpg|[[somelink]]]]
 * It recognises all the defined options. Unfortunately it's not quite as flexible as the currentparser in allowing ad hoc definitions like "thumb=$" etc.
 * Its treatment of options is basically, if the text doesn't match *exactly* a specified format (eg, "...|thumb=xxx|", then it's a caption. The following are teated as captions:
 * (possibly too harsh...)
 * It's case insensitive for all "magic word" matches, case sensitive otherwise.
 * It's case insensitive for all "magic word" matches, case sensitive otherwise.
 * It's case insensitive for all "magic word" matches, case sensitive otherwise.
 * It's case insensitive for all "magic word" matches, case sensitive otherwise.

Test cases
These test cases are parsed correctly: