Better ctrl+k

In OS X, `ctrl+k` is a system-wide command for deleting from the cursor to the end of line, and `ctrl+shift+k` will kill the entire line. This was being overridden by the `paredit_kill` and `paredit_kill_expression` key-mappings. The `paredit_kill` and `paredit_kill_expression` commands are mega-helpful, but even in LISP, there are times that you may want to kill to the end of the line while not inside of an expression.

My solution was simply to check whether or not the cursor is inside of an expression before using Paredit's function. If you're outside of an expression, the normal behavior for your platform will occur.

Thanks!