ENH: Add option to ignore any broken plugins.

Currently, if there are any broken plugins, the EggBasketPluginManager
just fails to return any plugins at all crashing an the application when
the application runs.  The added option which is off by default allows
the plugin manager to continue loading good plugins and returns them
while storing any errors for the application writer to act on.
TEST: Add test for ignore_broken_plugins option.
TEST: Undo side-effects which break other tests.
Adding python 2.6 egg so Travis CI tests pass.
Update as per PR review.

EggBasketPluginManager now has a single trait called on_broken_plugin
which can be callable which is called if there is an error loading a
plugin.  The callable is passed the entry point and `sys.exc_info()`
Update as per PR review.

The on_broken_plugin callable now is given the entry point and exception
and the default implementation simply raises the exception.
Merge pull request #40 from prabhuramachandran/gracefully_load_bad_plugins

ENH: Add option to ignore any broken plugins.