Updating Singularity Documentation.
Make Singularity embeddable into a bigger service.

Adds also an example on how to do it with a skeleton class.

There is probably some more documentation required; I will write some once this has really gelled.
another generic change for extensibility.
add configuration reference
tweaks
add mesos settings / TOC
caps
deployment section
Merge branch 'master' into update_docs

Conflicts:
	Docs/Singularity_API_Reference.md
more rework of docs
Merge pull request #314 from HubSpot/update_docs

Updating Singularity Documentation.
Merge pull request #295 from hgschmie/embed-singularity

Make Singularity embeddable into a bigger service.
link updates
rate limit mails

- rate limit is per mail type + request
- rate limit after a configurable # of mails in a given period
- lasts for a configurable amount of time
- notifies rate limit is in effect
- cleans up old metadata used to calculate rate limit
Merge branch 'rate_limit_mails' into hs_staging