Config restructure; Readme update; Pip functionality

Simplify charm configuration
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- Remove `code_archive_uri` option
- Make `code_archive` option take either a URL or a path within the charm
- Make `app_label` default to unit name, so it doesn't need to be set
- Create `code_archive`, `wsgi_application`, `apt_dependencies` and `nagios_index_content` config options so that the basic features of the charm are usable without editing `playbook.yml`
- Code is now stored in timestamped directories

* One potential issue with the above solution - the archive gets expanded into a new timestamped directory every time the config changes, regardless of whether the config has actually changed. We may want to add new logic to check whether the archive has changed or not.

Update the readme
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- Move informative commands out of Makefile and into README (hopefully readme has still remained quite simple)
- Simplify and resize headings
- Reword slightly

Pip functionality
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This is to support installing of pip requirements for a WSGI project. You provide the path to the `requirements.txt` file, and optionally a path to a pip-cache directory containing local caches of the dependency modules. 

- Add two new config options: `requirements_path` and `pip_cache_path`
- Update all README reference charms to trusty, as pip commands don't work in precise