Some slime asdf cleanups:
* Drop the unsupported :force hack from swank.asd
* Update some comments and docstrings
* Remove an obsolete #+asdf2
Merge pull request #76

Some ASDF cleanups.
Closes #78: `slime-edit-definition' on no symbol should prompt user.

* slime.el (slime-edit-definition): Fix bug introduced by changes
of 2013-12-30. Closes #78.