Chris Shoemaker's patch to add a "Recurrence" data type to the engine.

[David Hampton]

Log Message:
-----------
Chris Shoemaker's patch to add a "Recurrence" data type to the engine.
This should eventually subsume the FreqSpec data type.

Tags:
----
gnucash-gnome2-dev

Modified Files:
--------------
    gnucash/src/engine:
        Makefile.am
    gnucash/src/engine/test:
        Makefile.am

Added Files:
-----------
    gnucash/src/engine:
        Recurrence.c
        Recurrence.h
    gnucash/src/engine/test:
        test-recurrence.c