CRM-17860 - CRM_Utils_System_WordPress - Path lookup

CRM-17860 - CRM_Utils_System_WordPress - Path lookup

The implementations of `getDefaultFileStorage()` and
`getCiviSourceStorage()` depended on having the CMS bootstrapped, causing
failures in scripts that boot Civi first.

This patch computes those values by doing some arithmetic, e.g.
 * Civi source URL == CMS Base URL + relative path to source tree
 * Civi file URL == CMS Base URL + relative path to files dir

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* CRM-17860: More consistent, flexible handling of tests for extensions
  https://issues.civicrm.org/jira/browse/CRM-17860