Added GeneratorUtilities.prototype.isBaseDirExists function.

Prevent compound to create base directory when it already exists.

Today I just ran into problem when I have `blog` directory in my working directory.
I fired up `compound init blog && cd blog` and see this directory is kinda weird there are `Gemfile`, `Rakefile` and other Rails stuffs mixed with compound files.

So this commit fixed that.

P.S. I saw coding styles are not consistent, we could use jshint or some linter :smiley: