feat #327 improve error messages in assertEquals/NotEquals/JsonEquals

1. Allow usage of %1 %2 in assertEquals messages

%1 and %2 are shortcuts for value1 and value2 params passed to
%assertEquals and can be used as placeholders in the error message.

This commit implements the feature (Assert.js) and changes some of the
unit tests to use the new capability.

2. Fix the obvious issue with the error message of
'assertNotEquals'.

3. Write serialized versions of JSON entities in assertJsonEquals
in case there's no default error message passed.

Close #327. Close #328.