registered origin constraints with /info

min_ttl, max_ttl, default_ttl, and max_cdn_file_size are now
exposed in the /info endpoint under the 'cdn_origin' key.

In order to keep only one place for setting the default values
in the code, the defaults for these variables is set in a constant.

A functional test was added to check that these values exist and
return something castable to an int.

Updated the sample config to reflect the default value set by the code.
Merge branch 'add_info' of github.com:notmyname/sos into pr

Conflicts:
	sos/origin.py
Merge pull request #47 from dpgoetz/pr

Pr