Add method TaggedEC2Object.add_tags().

There is already a TaggedEC2Object.add_tag() method but it only supports
adding one tag at a time. The CreateTags api supports adding multiple tags
at once. So, when you want to add multiple tags you needlessly make multiple
round-trips to EC2 service. The add_tags method helps in avoiding that.
Fix behaviour of remove_tag when empty string is provided as value.

The documentation says that there is a distinction between a
value of '' and a value of None. The API docs say the same
thing. But the code was not following the docs:
    if value:
returns False for both None and ''.
Add tests for add_tag, add_tags, remove_tag methods of TaggedEC2Object
Merge pull request #2259 from mindprince/add_tags

Add method TaggedEC2Object.add_tags(). Fixes #2259.