CRM-15126, CRM-14949 - CRM_Utils_File - Fix dynamicResourcePath() and absoluteDirectory()

 - In dynamicResourcePath(), don't allow mixed-delimiters to produce double-delimiters (e.g. "files/civicrm/\dynamic")
 - Update absoluteDirectory() to work in Windows
Merge pull request #3869 from totten/master-win-dynjs

CRM-15126, CRM-14949 - CRM_Utils_File - Fix dynamicResourcePath() and absoluteDirectory()