Various CreditMemo related items:
1. :placed_on should be depreciated on the next major release. Replace
it with :txn_date as it is the standard across the models.
2. :place_on should be :as => Date
3. Dry up ensure_line_items_initialization by putting it in
BaseModel. Add ensure_line_items_initialization to CreditMemo.
It is .now not .new and use a more lenient regex.
Travis does not like + sign in utc.
Merge pull request #94 from minimul/credit-memo-items

Various CreditMemo related items: