checksums: use sha256 checksums everywhere

We'd like to use a consistent checksum scheme everywhere so that we can:

    a) incorporate archive checksums into our specs and have a
       consistent hashing algorithm across all specs.

    b) index mirrors with a consistent type of checksum, and not one that
       is dependent on how spack packages are written.

- [x] convert existing md5, sha224, sha512, sha1 checksums to sha256

var/spack/repos/builtin/packages/fr-hit/package.py

@@ -13,7 +13,7 @@ class FrHit(Package):
     homepage = "http://weizhong-lab.ucsd.edu/frhit"
     url      = "http://weizhong-lab.ucsd.edu/frhit/fr-hit-v0.7.1-2013-02-20.tar.gz"
 
-    version('0.7.1-2013-02-20', '3e8ea41ba09ab0c13e9973fe6f493f96')
+    version('0.7.1-2013-02-20', sha256='44dcfeb73106529fcefb02c017ec7b95b04b6523a2a57683b2bc905c142e62eb')
 
     depends_on('perl')
     depends_on('python@2.7:')