Add prelude-smart-open-line-above defun to prelude-core.el

Opens a line above the current line and indents it.

The counterpart to prelude-smart-open-line (bound to S-ret)

The new defun is bound to C-S-ret and is documented in the README.
Minor corrections to doc lines in prelude-core

They sound better grammatically.
Simplify implementation of prelude-smart-open-line-above

As suggested by bbatsov.
Merge pull request #291 from Abizern/add-to-prelude-core

Add to prelude core