Bug 22834: Add Koha::Plugins::Base->bundle_path

Bug 22834: Add Koha::Plugins::Base->bundle_path

This patch adds a 'bundle_path' method that return the base path for
assets included in the plugin. The idea is that it will be used for
calculation of the absolute path in the filesystem for static content.

To test:
- Apply the unit tests patch
- Run:
  $ kshell
 k$ prove t/db_dependent/Plugins.t
=> FAIL: The method is not implemented
- Apply this patch
- Run:
 k$ prove t/db_dependent/Plugins.t
=> SUCCESS: Tests pass!
- Sign off :-D

Signed-off-by: Arthur Suzuki <arthur.suzuki@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>