* Initial concept for pluggable storage (sans tests and docs)

* Add TLS storage docs, test harness, and minor clean up from code review

* Fix issue with caddymain's temporary moveStorage

* Formatting improvement on struct array literal by removing struct name

* Pluggable storage changes:

* Change storage interface to persist all site or user data in one call
* Add lock/unlock calls for renewal and cert obtaining

* Key fields on composite literals