making the base path configurable

Hi, 

i find it very convenient in my slush generators to create the target directory of new slush generated projects inside the generator itself. Otherwise many people (for example me ;)) are ending up generating content in a project base repository and not in the targeted new project specific one.

To accomplish this i added an optional parameter for the basepath, which can be used instead of the cwd. I hope you'll find this helpful too.

As a little side note. I was not able to run the tests successfully on my machine, do you have any hints there?

Thanks in advance, Falk