Bumping the nodejs-dev image to 0.10.24

and changing the default 'clean' target to *not* remove images, as this
likely results in pushes of incomplete tags such as happened recently
with build-essential.
Reinstating the image cleaning as its own task

plus some renaming to match make convention and documenting the
clean-images task.
Merge pull request #14 from modcloth/bumping-nodejs-dev-image

Bumping the nodejs-dev image to 0.10.24