add test of add/subtract method for operator overload
write pod about add/subtract method
Merge pull request #2 from Songmu/feature/add-subtract-docs

Feature/add subtract docs
Checking in changes prior to tagging of version 0.03.

Changelog diff is:

diff --git a/Changes b/Changes
index 4d2ebeb..2a3db72 100644
--- a/Changes
+++ b/Changes
@@ -1,5 +1,8 @@
 Revision history for Perl extension Time::Piece::Plus

+0.03    Wed Feb 13 06:05:00 2013
+        - override add/subtract method for providing interface like DateTime
+
 0.02    Fri Dec 10 01:17:50 2011
         - Add today method
         - yesterday and tommorow method more faster