Consider splitting appengine-magic to multiple libraries

[nukemberg]

while working on 1.8.x sdk support i encountered a classloader error in the dev server:

java.lang.ClassCastException: com.google.appengine.tools.development.ApiProxyLocalImpl cannot be cast to com.google.appengine.tools.development.ApiProxyLocal

This was caused by com.google.appengine/appengine-api-stubs and com.google.appengine/appengine-testing dependencies being copied into the war file. while it's simple enough to exclude these in appengine-prepare lein task, the correct solution would be to separate runtime dependencies from lein plugin and test dependencies. I suggest splitting appegine-magic to 3 libraries - lein-appengine-magic, appengine-magic and appengine-magic-testing so that users can add dependencies to lein profiles as needed.

[gcv]

This is probably a good idea. When I have time to work on appengine-magic, I'll probably do this.