Deprecate the Dataset.append() method

We have vstack and hstack for this. They are actually more functional.
It makes little sense to me to have this duplicated. OUr general
paradigm is to yield a new dataset whenever the content is modified,
append() is not following this scheme.
ENH: Use our own warning system for deprecation messages.
Merge pull request #132 from hanke/_tent/noappend

Deprecate the Dataset.append() method