Low barrier to entry

An emphasis for our backend system has been to offer a real low barrier to entry for adding new backend functions. Therefore we have tried to avoid unnecessary configuration files, just a module that you drop in each submodules backend directory with naming convention doing all the rest.

Documentation for each function is updated to indicate what backends are available. To do that though without importing a module, we have resorted to quick parsing of the function definitions of a module.

At the start of the execution, the source tree is scanned and scikits.image.backends is populated with all found backends. This can be used for backend exploration.

use_backend("opencv")
use_backend(scikits.image.backends.opencv)

Scikits.image already has a IO plugin system with very similar though slightly more complicated infrastructure, and in the future it may be wise to merge the two systems. This will require two tiers of backends one for visualization and one for processing.