Fix python2.6 deprecation warnings related to hashlib.
* bzrlib/osutils.py: Wrap md5 and sha imports to be compatible with python 2.4, 2.5 and above. Replace all sha.new() calls by sha() calls they are reputedly faster (not profiled).
* bzrlib/weave.py: Update sha import, fix use.
* bzrlib/transport/http/_urllib2_wrappers.py: Update md5 and sha imports, fix use.