rmtree(): use exponential increase in delay

Previously the dt = 1, 2, 3, 4, 5, now it is dt = 1, 2, 4, 8, 16.
Increase max_retries to 6

So now the final dt=32s.
replace custom logging with python standard library
Python 2.6 fixes
Only use two loggers, root and package

This simplifies the logging: The root logger is the default, and the
package logger contains the package name in square brackets. Also, a
custom formatter is added for level-dependent formatting.
Fix file discovery if the profile contains "use:"

The hook filename (if any) is moved into the PackageYAML class which
already encapsulates filesystem information.
Merge pull request #274 from hashdist/use_packagedir_files

Fix hook file discovery if the profile contains use:
Merge pull request #262 from vbraun/logging

replace custom logging with python standard library
Merge pull request #265 from hashdist/rmtree

Make rmtree() more robust