Fix installation of non-live extension versions

The path to 'tar' is spelled "configuration.executables.TAR", not
just "TAR".
Add <title> elements on some more pages

Was missing on /config, /branches and /tutorial (when not used to
show a specific tutorial, in which case the turorial's title was
set as the document's title automatically.)
Kill some unnecessary code duplication
Only return True in install("blah") if it actually installed something
Drop 2x assigned-but-not-read local variables 'all_ok'
Drop assigned-but-not-read local variable "started"
Fix two typos in README.md
Use the term "system group" instead of "Unix group"

(since the former is the terminology that install.py uses)
Drop assigned-but-not-read local variable "review_arg"
Merge pull request #13 from mo/minor_fixes_20121031

Minor fixes 20121031
Merge pull request #12 from mo/fix_bcrypt_restart_install_bug

Only return True in install("blah") if it actually installed something
Updated CONTRIBUTORS
Use --etc-dir argument if provider, otherwise ask for path

The etc_dir was effectively hard-coded to /etc/critic, for no good
reason.
Retrieve proper review id instead of using undefined local variable
Use module prefix for unlink() calls
Use correct variable name in error message (s/branch_name/branch/)
Import OperationFailure since it's used in this file
Use module prefix for User.fromId() call
Drop module prefix for locally defined class
Import re module needed for re.sub()