Fixes #215 changed timer to only reset when an update is run, not just an attempt to update

Before, I could still lock the data forever by calling faster than once per second.
Now, I can call as fast as I want an the number of calls to update is still controlled by cached_time.
I get the following results using java-glances to call every 300ms:
```
Connecting to glances server: 'http://localhost:61209/RPC2'
Time: 10:32:48
Testing getNow()
[current time]: Tue Mar 19 10:32:49 CST 2013
Testing getNow()
[current time]: Tue Mar 19 10:32:49 CST 2013
Testing getNow()
[current time]: Tue Mar 19 10:32:49 CST 2013
Testing getNow()
[current time]: Tue Mar 19 10:32:49 CST 2013
Testing getNow()
[current time]: Tue Mar 19 10:32:50 CST 2013
Testing getNow()
[current time]: Tue Mar 19 10:32:50 CST 2013
Testing getNow()
[current time]: Tue Mar 19 10:32:50 CST 2013
Testing getNow()
[current time]: Tue Mar 19 10:32:50 CST 2013
Testing getNow()
[current time]: Tue Mar 19 10:32:51 CST 2013
Testing getNow()
[current time]: Tue Mar 19 10:32:51 CST 2013
Testing getNow()
[current time]: Tue Mar 19 10:32:51 CST 2013
Testing getNow()
[current time]: Tue Mar 19 10:32:51 CST 2013
Testing getNow()
[current time]: Tue Mar 19 10:32:53 CST 2013
```