CRM-12556 - api/v3 - Identify contacts by user name

Example: drush civicrm-api contact.get id=@user:demo

I read "System_Drupal::loadUser()" to get a starting point for writing
"System_Drupal::getUserId()"; however, only Drupal defines loadUser(), so
there was no starting point for the other CMSs.  This will degrade in a sane
way on other CMSs.

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* CRM-12556: Assign api_key for a user via command-line
  http://issues.civicrm.org/jira/browse/CRM-12556
CRM-12556 - Treat string "202" and number 202 the same

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* CRM-12556: Assign api_key for a user via command-line
  http://issues.civicrm.org/jira/browse/CRM-12556
CRM-12556 - Update to match newer parameter name

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* CRM-12556: Assign api_key for a user via command-line
  http://issues.civicrm.org/jira/browse/CRM-12556
CRM-12556 - api_v3_ContactTest::testCreateNameIndividualOldKeys2 - Fix regression

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* CRM-12556: Assign api_key for a user via command-line
  http://issues.civicrm.org/jira/browse/CRM-12556
CRM-12556 - Add unit tests for fetching contact by username

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* CRM-12556: Assign api_key for a user via command-line
  http://issues.civicrm.org/jira/browse/CRM-12556
Merge pull request #1209 from totten/contactid-userid

CRM-12556 - api/v3 - Identify contacts by user name