Fix the user set cursor being overridden

Function `org-present-read-only' sets the value of the cursor to nil
effectual throwing away the user set configuration. Using a temp
variable `org-present-cursor-cache' to fix that.
Kill trailing white space
Exit read-only mode before exiting the minor mode
Merge pull request #6 from jaseemabid/master

Fix the user set cursor being overridden