add offset to date of 'riak-cs-gc status'

`riak-cs-gc status`'s output looks like to need to convert epoch time to UTC.

#### before fix

```
% ./riak-cs-gc batch & ./riak-cs-gc status
[1] 6732
Garbage collection batch started.
A garbage collection batch is in progress
  The current garbage collection interval is: 900
  Last run started at: 00430917T132740Z                <------------
  Current run started at: 00430917T132742Z             <------------
  Next run scheduled for: 00430917T134240Z             <------------
  Elapsed time of current run: 0
  Files deleted in current run: 0
  Files skipped in current run: 0
  Files left in current run: 0
[1]  + 6732 done       ./riak-cs-gc batch
```


#### after fix

```
% ./riak-cs-gc batch & ./riak-cs-gc status
[1] 96703
Garbage collection batch started.
A garbage collection batch is in progress
  The current garbage collection interval is: 900
  Last run started at: 20130917T132621Z                <------------
  Current run started at: 20130917T132623Z             <------------
  Next run scheduled for: 20130917T134121Z             <------------
  Elapsed time of current run: 0
  Files deleted in current run: 0
  Files skipped in current run: 0
  Files left in current run: 0
[1]  + 96703 done       ./riak-cs-gc batch
```