Support Kanso Package installation

The repo http://kan.so/packages has a lot of good stuff that one can use in any couchapp.

For now, we would want to have the following command in erica

```
erica install couch-tokens
```

Which would do the following:
- download the package from the urtl http://kan.so/packages/_db/couch-tokens/couch-tokens-0.0.1.tar.gz
- extract the packages into a temp dir
- copy the extracted contents into the 'ddoc' directory
- ignore kanso.json and readme files.

The above would not provide complete kanso integration, but would be what most people would need.

Start at the erica CLI command entry point and inspect how existing commands handle downloads, temporary directories, extraction, and copying into a couchapp. Done means `erica install couch-tokens` retrieves the specified archive, extracts it, copies its contents into `ddoc`, and excludes kanso.json and README files.

Written by the indexing model from the issue text.