Montecarlo simulator can now simulate two opposed dice.

Also, empty strings now count as "0".
Windows now start at 100,100.

Otherwise, they were sometimes difficult to operate with (e.g. Unity).
Empty fields are set to 0 in simple roller.

See commit 9f18897.
Reorganised tests while looking for a bug
Fixed typo that resulted in bug for dice expressions like "2D100".
Added comments on TODO.
The parser now admits the minus sign.

This allows expressions like 1d6-2.

Additionally, I have put all dice-expression-related exceptions under
the same class (IllegalDiceExpressionException) to facilitate error
handling.