Add Config Options to Expire Content Based on LastAccess/LastModified Time

This change adds configuration options to expire content based on
LastAccessTime and/or LastModifiedTime. Each allows two different
forms of specification:
  - Expire content older than some specific date.  (Absolute)
  - Expire content older than some number of days. (Relative)

The configuration parameters are:
filesystemadaptor.lastAccessedDate - Absolute last accessed time expiration
filesystemadaptor.lastAccessedDays - Relative last accessed time expiration
filesystemadaptor.lastModifiedDate - Absolute last modified time expiration
filesystemadaptor.lastModifiedDays - Relative last modified time expiration

Values for the Relative forms of configuration are positive integer
number of days.  For instance:
   filesystemadaptor.lastAccessedDays=365
would expire content that had not been accessed for a year.

Values for the Absolute forms of configuration are ISO8601 date
only format strings: yyyy-MM-dd.  For instance:
   filesystemadaptor.lastModifiedDate=2010-06-01
would expire content that had not been modified since the beginning
of June 2010.

Code Review: http://codereview.appspot.com/96330043