If Rails cache is set to Memcached, use it to provide global locking and shared last_scaled times

Closes issue #3
Added Gemfile and Gemfile.lock
last_scaled(=) need to be public methods
Updated rspec to 2.11, replaced rr with rspec mocks
Wrote a spec to test locking behaviour. Also added 'debugger' to Gemfile
for 1.9.x
Use gem name in cache key ('_' => '-')
Merge pull request #5 from ndbroadbent/memcached_locking_and_last_scaled

If Rails has Memcached configured, use it to provide global locking and shared last_scaled times