Man page: Add description about using None

Fixes #181
Plugins: Classes should inherit from object
Merge pull request #185 from pynag/fix-documentation-reference-to-None

Man page: Add description about using None
Merge pull request #187 from pynag/feature-plugin-classes-new-style

Plugins: Classes should inherit from object
Fix for directories inside log archives

This patch fixes a problem where an unhandled IOError would be
raised if a non-file exists under archive log directory.

Fixes #498
Add a dummy directory to log archives
Fix $HOSTDISPLAYNAME$ not being resolved correctly

HOSTDISPLAYNAME should refer to self.display_name but was
defaulting to self.displayname

In addition if it is not defined it should refer to self.host_name

I also did a little bit of clean up of the standard macro look up.

it still needs more work, but this change is only for the better.

contact macros remain fairly unusable and the list of standard macros
in macros.py is fairly incomplete.
Refactor Perfdata things into pynag.Utils.metrics
Refactor GitRepo into its own module Utils.git
Refactor PynagError into Utils.errors
Refactor send_nsca into Utils.nsca
Clean up imports and linebreaks in Utils
Refactor synchronized and cache_only to decorators
Clean up imports and ordering in Utils
Refactor reconsile_threshold into Utils.metrics

+ Minor fixes
Move errors.py to top-level
Refactor reconsile_threshold>new_threshold_syntax

This function belongs there because it only supports
the new threshold syntax.

Also renamed to more approprately 'convert_to_classic_format'
Fix broken imports for errors module
Explicitly import PynagError

This is needed because there are still unresolved cross-imports
in pynag and explicitly importing PynagError fixes parts of the code
that Utils was import but at the same time those modules were doing:

'from pynag.Utils import PynagError'

This refactor, and a few other changes will make it possible for
us to get rid of those cross imports.
Merge pull request #188 from pynag/fix-directory-in-archives

Fix for directories inside log archives