Add Paypal donate button
Add cider-undef with C-c C-u binding

Adds an undef command for removing a symbol from the current namespace.
Merge pull request #721 from hugoduncan/feature/add-undef

Add cider-undef with C-c C-u binding
Add a missing dot
Mention PayPal donations in the contributing section
Minor changes
Document the release policy
Remove assert in cider-util.el
Add requires to cider-stacktrace

cider-stacktrace was not loading correctly due to missing requires on
cider-util and cider-client.