Use abspath when building cakedir from static_dir

This change allows for using relative path tricks like `..` in the `cakeparent` parameter. This was needed in my case because my app is organized with the webapp part as a submodule to the full package.

For example, my app is in 

    /home/ubuntu/projectdir

The coffeescript files are in 

    /home/ubuntu/projectdir/package/subpackage/webapp/static/coffee/

I tried initializing Flask-Cake with the `cakeparent` keyword argument as:

    cake = Cake(app, ['build', 'compress'], cakeparent='../package/subpackage/webapp/static/coffee/')

This resulted in the error:

    IOError: [Errno 2] No such file or directory: '/home/ubuntu/projectdir/static/../package/subpackage/webapp/static/coffee/Cakefile'

The patch fixed the issue because it properly expanded the `..` so that it looked for the Cakefile at:

    /home/ubuntu/projectdir/package/subpackage/webapp/static/coffee/Cakefile