Adds test_cli.Cp.test_cp_copies_dir
tahoe cp -r now copies the top level directory with its children
Fix tests which relied on the old behavior of tahoe cp -r
Documentation updates
CLI.rst: expand the 'cp -r' subdirectory example
Merge branch markberger/712-tahoe-cp-doesnt-copy-dir

Closes ticket:712 (trac).
Closes tahoe-lafs/tahoe-lafs#56 (github PR).