Added bit string requirement to setup.py
Added bitstring requirement to tests
ntfs plugin now uses bitstream module
Added ntfs plugin test
Added singleton utility class
Converted Singleton class to metaclass

Also added _drop method, used in unit tests to clear instance dictionary
FilesystemDetector is now singleton
Removed __init__ form singleton
Added _clear_plugins to detector for unit tests
Fixed lint warning
Finished ntfs_plugin test
Added bitstring prereq. for tox.ini tests
Merge pull request #15 from dariusbakunas/implement_tests_for_ntfs_plugin

Implement tests for ntfs plugin