Add reset_expire

[alex88]

This is something that hopefully helps me solve #22.
Basically it lets the user add a safe guard in case the process that has acquired the lock dies (e.g. due SIGKILL) by being able to renew the lock expiration to a later time. I imagine the usage to be:

# Create the mutex with a very low expire time, this is useful so in case the process dies, the mutex isn't
# locked for more than 10 seconds (in a long process as in my case the mutex is locked for 5
# minutes so in case of process failure it generates a very long delay)
mutex = RedisMutex.new(:test_lock, :expire => 10, :block => 0)
mutex.with_lock do
  data_to_be_processed.each do |row|
    make_external_request(row) # Hopefeully this takes less than 10 seconds
    mutex.reset_expire                # Get another 10 seconds to work on the next row
  end
end

let me know if it's something you're interested in, in that case maybe the get+set has to be done in a single redis transaction and I should also add some docs in the readme. Otherwise I'll just keep the fork for our project

[kenn]

This should check the ownership of the lock by get == @expires_at.to_s instead of just locked?