change incanter.core to a consistent style, for real this time

This pull request is more or less like https://github.com/liebke/incanter/pull/208 but with the agreed upon docstring from https://github.com/liebke/incanter/issues/204.
For docstrings consisting of only one line, I left them in their single line.

Sorry it took me a month, I hope to go through the rest of the modules next, and then maybe take a look at the examples.