Upgrade of cookbook

Hello.

I've started working on upgrading your cookbook and would like to share this with other.
Not sure, it is should be merge to master or to separate branch before all thinks done.
Anyway, here is brief description of changes:
1. Applied many skeleton files (Gemfile, Berksfile, Rake Kitchen, etc)
1. `rake` by default tests cookbook using rubocop, foodcritic and chefspec
1. Node attributes have been converted from symbols to string
1. Added simple specs as example for testing
1. Cookbook by default installs "default"-plugins, defined in `node['collectd']['default_plugins']` array
1. Introduced TestKitchen with Ubuntu and Debian
1. Added few integration tests
1. Definition has been converted to standalone LWRP
1. Added Fake LWRP `collectd_python_plugin` which actually modifies original `collectd_plugin[python]` resource
1. `collectd::client` should not fails when `servers` is empty, only notice and skip network configuration
1. Added integration tests for Fake LWRP `collectd_python_plugin`
1. Integration tests checks that collectd is configured properly by executing `collectd -t` command