Remove stats section for 2.0

Right now also the stats are not working in Fauxton and we
decided to remove it right now.

In a serious production setup stats are provided by external
tools like collectd, graphite, monit and friends - we include
couchdb-folsom for that in 2.0.

Discussion on ML:
https://www.mail-archive.com/dev%40couchdb.apache.org/msg37245.html

Closes COUCHDB-2429
Fix bulk-delete on all-doc-page

On Couch 1.6 `/_bulk_docs` returned `ok: true` when the request was an
success. On 2.0 the ok property is missing:

Example success message on 2.0:
```
[{"id":"Brocket","rev":"6-b6fa9e703c3eb92aa9c3a49cedf1e8c2"}]
```

Example error message on 2.0:
```
[{"id":"Blaggie-System","error":"conflict",
  "reason":"Document update conflict."}]
```

Steps to reproduce the error in Fauxton:

 - select 1 document in the all_docs screen
 - click the trash icon to delete it
 - click ok to delete it
 - you are able to click the trash icon again and if you do, you
   get an error

Closes COUCHDB-2462
Add Nightwatch functional browser tests to test Fauxton

This commit adds a basis for using Nightwatch's functional
browser testing environment to test Fauxton.

Closes COUCHDB-2369
add license headers
Updated style guide to include HTML and CSS sections

Closes COUCHDB-2416
Feedback from @robertkowalski