Allow input multiline for some fields.

Allow input multiline for the following events offers fields.

* Coupon, Preview
    - Chinese name
    - English name
    - Spec
    - Item code
    - Extra note

* Exhibition
    - Title

* Announcement
    - Title
    - Content (It was allowed using multiline)
Process user input data (Remove '\r')

The purpose of this commit is to make multiline string saving in datastore
consistently. Try to always using unix EOL style (\n) in multiline string.

The following description is based on the communication happened between client
(chromium) and servers (dev or prod):

    - When client send POST request with enctype=multipart/form-data, the
      mulitline string is sending with Windows EOL style (\r\n) to server.
      (dev server also has a bug, which preppend an extra \r in the received
      EOL. ==> \r\r\n)

    - When client send PUT request with x-www-form-urlencoded content type,
      the multiline string is sending with Unix EOL style (\n) to server.
Add coupon item discount unit.
Avoid exception.

When publishing event which contains old item missing required field, assign
empty value to this field.
Add uploaded image dimension properties.

Retrieve height and width from images service.
When Image is constructed from blob_info for blob_key, before we can retrieve
the height and witdh information, we need to do some transformation first.
If Image is constructed from byte string, we can directly retrieve dimension.