Initial pass at catching an invalid identity crisis caused by wp-config

Initial pass aimed at updating the identity crisis check to include a check for constants
that can be defined in wp-config that will cause a mismatch between the url wpcom expects
the wporg site to have and the actual url used. Potentially fixes #1006
Fix oops on continue always firing
Add tests for id crisis check.
Fix id crisis force ssl check.

Check constant definedness properly. Made the function an instance method, because why not, and easier stubbing.
Merge pull request #1021 from Automattic/id-crisis

Resolve identity crisis when using FORCE_SSL_ADMIN