Removes continuous.png file after specs

The "continuous.png" file is being created during some specs and ends up
in the repository. This shouldn't happen.
Removed graph calls in specs

They're unnecessary and will be substituted by a further spec.
Test figure creation

Simply creates a tree, trains it and produces a PNG figure, testing to
confirm that no exception was raised and that the image is really there.
Using a constant for the filename
Merge pull request #16 from agarie/cleanup_specs

Removes continuous.png file after specs