Implements CDN Manager and CLI support (#47)

Provides good basis for CDN support that should cover a vast majority of use cases for the time being. Passes all tests and PEP8 checks.

Operations supported are as follows:
```
usage: sl cdn [<command>] [<args>...] [options]

Manage CDN accounts and configuration

The available commands are:
list           List CDN accounts
detail         Show details for a CDN account
load           Cache one or more files on all edge nodes
purge          Purge one or more cached files from all edge nodes
origin-list    Show origin pull mappings on a CDN account
origin-add     Add an origin pull mapping
origin-remove  Remove an origin pull mapping
```

Thanks to @underscorephil for the majority of test coverage and a few other fixes. Some of these commits were merged and squashed together due to rebasing against master and against Phil's test branch, but the history all looks intact.