Use python_pip resource instead of execute

There was a lot of duplication across recipes using the `execute` resource to do a source install for each component, whisper, carbon, and graphite-web. This is part of gradual changes toward a more a library style cookbook.

This switches to using the `python_pip` resource provided by the python cookbook. The result is much cleaner recipes for whisper, carbon and web.

Source installs are still possible if desired using the new `node['graphite']['source_install']` attribute, which will use the url attributes for each component:

```ruby
default['graphite']['carbon']['url'] = "https://github.com/graphite-project/carbon/zipball/master"
default['graphite']['graphite_web']['url'] = "https://github.com/graphite-project/graphite-web/zipball/master"
default['graphite']['whisper']['url'] = "https://github.com/graphite-project/whisper/zipball/master"
```

Tested on centos 6 and ubuntu 12.04