Add a few functions to plugin utilities.
Fix bug in PluginsLoader.load function.

If a plugin was enabled but later removed from disk then the program
would crash. KeyError exception occured in

plugin = self.all_plugns[UID]

finally block tried to use  finding_failed variable which was not
defined because it was initialized after the above line. This
was causing another exception that was not caught.

Added logger and some log messages.

Added handler for KeyError exception.
Fix untrack call in PluginsLoader.

untrack function was called with plugin object but were supposed
to be called with UID. This was crashing the program because in
some cases plugin did not exist.

Gave PluginsLoader logger object.
Add functions for inserting actions in the context menu.

At the moment there is only function for adding to the leaf menu.
Merge pull request #19 from alexey-naydenov/plugin_helpers

Utilities to help with plugins
Pandas time series support added to time series plugin.

The plugin has been simplified and refactored.
<Thanks To> tab of the About ViTables dialog has been updated.

Alexey Naydenov has been added to the page as contributor.
Time series plugin support for soft/external links has been fixed.

Examples of PyTables files containing soft/external links have been
added to the examples directory.
The plugin utilities module has been updated.

Some functions have been renamed to more descriptive names. Docstrings
have been updated to show that some functions work well with both
QtGui.QAction and QtGui.QMenu objects. The function insertInMenu has
been added to the module.
Fixed a bug in the Export to CSV plugin.

Also the addEntry method has been refactored to use functions from the
plugin_utils.py module.
The Import from CSV plugin uses the plugin utilities module now.

The addEntry method has been refactored and now uses functions defined
in plugin_utils.py