Fixed make realclean.

Now removes main.exe.
Null type is now a hard-coded type. Made minor change to Pokemon.json format.

type2 is no longer a requred field. When it is missing, the second type will be set to
a null type with no modifiers. Improved the checking in the type parser.
Added documentation to json files.

Comments in each json describe the meaning of each variable.
Minor changes to readme.

Added an instruction in Windows building for clarity. Fixed grammar.