Fix broken callback in saveImage

saveImage only calls the callback method under error conditions.  Fixed
so that it also calls it when saveImage successfully writes the image
file to disk.

Callback was not being checked for presence, so if it wasn't passed in
an exception would be thrown. Fixed to provide a dummy callback if the
caller didn't specify one.

Removed spurious console logging on successful image write and fixed
some formatting in writeFile.
Update readme.md for correctness and formatting

The current implementation of saveImage does not just take a `figure`
from `getFigure`.  It expects an associative array where one of the
elements is a `figure` and the other elements specify image format,
width, and height.  Updated the documentation to reflect this.  The
right fix would be to take in a `figure` to `saveImage` along with an
options object specifying format, width, and height, to make the API
easier to understand. That would be a breaking change, though, so will
need discussion first.
Update getFigure and saveImage examples to reflect current APIs' behaviors

The examples were out of date with respect to API names and
functionality.  Updated and verified they work.
Simplify callbacks

In a couple places we can just callback to the original callback passed
into the method since we're not doing anything interesting with the
results.
Merge pull request #12 from nattress/master

Fix saveImage bugs and update documentation