add support for nested blocks

this allows the construction of more complex statements, like:

    %CRIT > 10% : {
        CRIT > 20 : CRIT;
        %OK  > 85%: OK;
        WARN;
    }

In the absence of default, it will fall through.

These blocks can be nested arbitrarily deep.

A ';' is needed after the block if followed by another statement.
small copyediting
add support for the != operator
test for nested blocks
document nested blocks
Changes for version 0.002