Add Apache httpd-style common log access logging.

This module introduces a new log file couch_access.log in the spirit of
Apache httpd's access.log.

The log format is specified here:

http://en.wikipedia.org/wiki/Common_Log_Format

The module defaults to the more commonly used `extended` format,
but can be configured to use the standard log format as well.

This is a new module rather than an extension to the couch_log
module to allow easy addition. Ideally, this will be a plugin in the future.

A note for chunked responses, the response body length reported is
"chunked" rather than the number of bytes. The case is unspecified
in the format description. Maybe the fix is keeping track of bytes
sent in a chunked response, but I didn't want to make the first stab
at this more complicated that it has to be. I'm happy to adjust.

See the full diff here:

https://github.com/janl/couchdb/commit/614091e49380b3afab3c34ff65e6d87623059b8b

I'm not proposing this to be added to 1.1.x, but if anybody wants it, there's a patch here:

https://github.com/janl/couchdb/commit/508af053257c074ec37c9bd658479cc21b8d20d4

(The only difference is the specification of the couch_access_log module on startup, as that
moved from couch_server_sup.erl to couch_primary_services.erl)