Change optional qty to optional since date for get_note_list()

get_note_list() was modified to support an optional quantity as an
argument to avoid having to retrieve the entire note list. This was
useful if you have a large number of notes. The intention was that
you could use it to retrieve the last 10 notes, etc. However, the
API does not guarantee the order in which the note index is
retrieved, therefore limiting the list returned to 10 notes, for
example, does not guarantee you'll get the 10 most recent notes -
it could be the 10 oldest.

The API does allow for a "since" parameter though, so it makes more
sense to use a date as an optional argument to get_note_list as a
way of limiting the number of notes retrieved.

The date has to be a string in YYYY-MM-DD format. Any other date
format will result in a ValueError exception which is caught, but
then passed meaning the "since" parameter will be ignored and all
notes will be retrieved.

- Changed optional qty argument to since argument
- Changed inline docs to suit
- Changed README to suit
- Removed tests specific to optional qty argument. Do not know how to
  add suitable test for since parameter.
Increase qty to max allowable by API to speed up retrieval

especially when there is a large number of notes
Merge pull request #9 from atomicules/develop-list-since

Change optional `qty` argument to get_note_list() to `since` date string