Add start and end time options for the report period.

These options, --start-time and --end-time, take ISO 8601 date/times (e.g., '2011-03-02T14:00:00-05:00'), and can be used to set any arbitrary reporting period you want.  The "previous" period will be the same length, one week earlier.  (Note all the labels say "vs. last week" for the percentage change values, but that should be roughly accurate for most uses.)

I added this because I noticed that using the current rotation period only works if no irregular exceptions are set. If you have a weekly rotation and someone sets a two-day exception, the report will only cover the two-day period versus the same two days a week ago.  Not as useful.  So, this works around that problem for now.

rotation-report.rb

@@ -48,8 +48,20 @@
     options[:rotations_ago] = rotations_ago
   end
 
+  options[:start_time] = nil
+  opts.on('-s', '--start-time DATETIME',
+          "Start of report (ex: '2011-03-02T14:00:00-05:00', defaults to current)") do |time|
+    options[:start_time] = time
+  end
+
+  options[:end_time] = nil
+  opts.on('-e', '--end-time DATETIME',
+          "End of report (ex: '2011-03-09T14:00:00-05:00', defaults to current)") do |time|
+    options[:end_time] = time
+  end
+
   options[:campfire_message] = false
-  opts.on('-m', '--campfire-message', "Paste the results as message in a Campfire room") do
+  opts.on('-m', '--campfire-message', "Paste the results as message in configured Campfire room") do
     options[:campfire_message] = true
   end
 
@@ -94,8 +106,17 @@
     rotation = policy.css("td.rotation_properties div table tr").each do |row|
       if row.css("td")[0].text =~ /On-call now/i
         period_offset = ONE_WEEK * options[:rotations_ago]
-        current_start = Chronic.parse(row.css("td span")[0].text) - period_offset
-        current_end   = Chronic.parse(row.css("td span")[1].text) - period_offset
+        if options[:start_time]
+          current_start = Time.xmlschema(options[:start_time]) - period_offset
+        else
+          current_start = Chronic.parse(row.css("td span")[0].text) - period_offset
+        end
+
+        if options[:end_time]
+          current_end = Time.xmlschema(options[:end_time]) - period_offset
+        else
+          current_end = Chronic.parse(row.css("td span")[1].text) - period_offset
+        end
 
         # Shifts are either one day or one week (currently, at least).
         # For a week-long shift, we want the previous full week. For a day