Rewrote constructor (new syntax). Adapted initialization routine.
Rewrote append method.
Fixed header doc.
Rewrote char method.
Rewrote ne, eq, le, ge, lt and gt methods.

These methods overloads ~=, ==, <=, >=, < and > relational operators.
Fixed error identifier.
Added (deep) copy method.
Rewrote the minus and plus methods.
Rewrote colon (:) method.
Rewrote display and disp methods.
Fixed bug.

freq member was not defined when both inputs were dates objects.
Rewrote double function.

Also changed unit tests. The syntax qq = dates('Q'); qq(1950,1) does not
work anymore, most likely because of inplace modifications...
Added error message id (in assert).
Rewrote horzcat method.
Rewrote intersect function.
Rewrote isempty method.
Rewrote isequal function and added unit tests.
Rewrote length method.
Rewrote max function and fixed unit tests.
Rewrote min function and fixed unit tests.