Added support of expression "end of month"

__end of month__ or __end of month at <time>__ - returns sequence of
the latest day for each month.

Also removed word “every” from regexp to allow specify expression in
flexible form like “end of every month” or “every end of month”
Introduced decorator for filters

ach function that starts with _filter should a generator, that yield
value  based on some conditions.

    Function like:

    >>> def _filter_monday(regexp_result):
    ...   monday = <process_regexp_result>
    ...   def f(gen):
    ...     for v in gen:
    ...       if v == monday:
    ...         yield v
    ...   return f

    might be defined in more generalized form:

    >>> @generic_filter
    ... def _filter_monday(regexp_result, item):
    ...   monday = <process_regexp_result>
    ...   return item == monday
Remove multiple function definition from _apply_time
reduced nr of lines in _get_xs_in_month
Merge pull request #5 from vasilcovsky/generic_generators

Gladly, sorry for not noticing the request sooner.