Added CHECKSUMS verification with --verify option.

Oriignally written as a plugin, now ported to the core.
Verify SIGNATURE file with cpansign command
suppress warnings for CHECKSUMS
Merge branch 'verify-dists' into devel
Checking in changes prior to tagging of version 1.5021.

Changelog diff is:

diff --git a/Changes b/Changes
index 679ca03..dd8e565 100644
--- a/Changes
+++ b/Changes
@@ -1,5 +1,10 @@
 See http://github.com/miyagawa/cpanminus/ for the latest development.

+1.5021 Thu Jan 31 00:42:28 PST 2013
+   [Improvements]
+      - Added new --verify option that enables verifying CHECKSUM and SIGNATURE for distributions
+        from CPAN/PAUSE
+
 1.5020 Tue Jan 29 10:29:08 PST 2013
    [Bug Fixes]
       - Fixed a bug in --cascade-search that was checking a wrong version from 02packages file (Bryce Baril)