Added some tests for `cygapt.utils.rmtree` function

Clean up following specification paths iteratively or not:
    - No exists
    - No read permission
    - Symlink to a file
    - Symlink to a directory
    - Broken symlink
Fixed `cygapt.utils.rmtree`
Merge pull request #19 from alquerci/fix/utils/rmtree

Improved test suite for rmtree