Generic filters

Summary:
1. Introduced decorator generic_filter (see below)
2. Multiple function definition in \_apply\_time reduced by functools.partial
3. Update for \_get\_xs\_in\_month
4. Filter \_end\_of\_month renamed to \_filter\_end\_of\_month
5. added support of expression "end of every month" (see prev. pull request)

Generic filters:

Each 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