problem with "webstyle" push

This is a follow up ticket to #50

It works nicely do deply a static HTML app with `erica push appname`. But I need to set the name of the design doc, is that possible?

Here's what I got

```
$ erica push appname
==> www (push)
==> Successfully pushed. You can browse it at: http://127.0.0.1:5984/appname/_design/www/index.html
```

here is what I want

``` diff
$ erica push appname
==> www (push)
- ==> Successfully pushed. You can browse it at: http://127.0.0.1:5984/appname/_design/www/index.html
+ ==> Successfully pushed. You can browse it at: http://127.0.0.1:5984/appname/_design/appname/index.html
```

when I add a `_id` file and run `erica push appname`, I get

```
$ echo "_design/appname" >> _id
$ erica pushappname
==> www (push)
ERROR: push failed while processing /path/to/www: {'EXIT',{ucs,{bad_utf8_character_code}}}
```

any idea?

Start at the `erica push appname` entry point and inspect how the webstyle design-document name and `_id` file are processed. Reproduce the command and error shown in the issue; done means the requested design-document URL can be selected without the reported push failure.

Written by the indexing model from the issue text.