Add age searching

Support for specifying relative time periods (the format the Gerrit web
 interface uses) and also absolute time
Added Joda Time library for datetime parsing assistance
Add age searching documentation to README
Output the AgeSearch in the same format it came in
SearchKeyword: Throw an exception if the name is invalid

Change-Id: I7d3ffbd9f03e0cbbf3aea38f3ac8e526d4718558