Add timestamps to calculation summary output

Addresses https://bugs.launchpad.net/openquake/+bug/1129271

The formatting of calculation and output summaries has changed as well. It's cleaner and more consistent now. For example:

<pre>
$ bin/openquake  --lhc
calc_id | num_jobs | latest_job_status | last_update | description
1 | 1 | pending | 2013-02-19 09:25:57  | Simple Fault Demo, Classical PSHA
2 | 1 | pending | 2013-02-19 09:26:05  | Simple Fault Demo, Classical PSHA
3 | 1 | failed | 2013-02-19 09:26:06  | Simple Fault Demo, Classical PSHA
4 | 1 | failed | 2013-02-19 09:26:07  | Simple Fault Demo, Classical PSHA
5 | 1 | failed | 2013-02-19 09:26:08  | Simple Fault Demo, Classical PSHA
6 | 1 | failed | 2013-02-19 09:26:09  | Simple Fault Demo, Classical PSHA
7 | 1 | failed | 2013-02-19 09:26:10  | Simple Fault Demo, Classical PSHA
9 | 1 | failed | 2013-02-19 09:26:22  | Simple Fault Demo, Classical PSHA
24 | 1 | successful | 2013-02-19 09:27:16  | Simple Fault Demo, Classical PSHA
10 | 1 | pending | 2013-02-19 09:26:22  | Simple Fault Demo, Classical PSHA
11 | 1 | failed | 2013-02-19 09:26:31  | Simple Fault Demo, Classical PSHA
12 | 1 | successful | 2013-02-19 09:26:32  | Hazard Map test job, 1 realization, no mean/quantile
25 | 1 | successful | 2013-02-19 09:27:28  | Simple Fault Demo, Classical PSHA
</pre>

<pre>
$ bin/openquake --lho 75
id | output_type | name
218 | ses | ses-coll-rlz-104
219 | gmf | gmf-rlz-104
220 | ses | ses-coll-rlz-105
221 | gmf | gmf-rlz-105
222 | complete_lt_ses | complete logic tree SES
223 | complete_lt_gmf | complete logic tree GMF
224 | hazard_curve | hazard-curve-rlz-104-SA(0.1)
225 | hazard_curve | hazard-curve-rlz-105-SA(0.1)
226 | hazard_curve | hazard-curve-rlz-104-PGA
227 | hazard_curve | hazard-curve-rlz-105-PGA
228 | hazard_curve | mean curve for SA(0.1)
229 | hazard_curve | quantile curve (poe >= 0.15) for imt SA(0.1)
230 | hazard_curve | quantile curve (poe >= 0.5) for imt SA(0.1)
231 | hazard_curve | quantile curve (poe >= 0.85) for imt SA(0.1)
232 | hazard_curve | mean curve for PGA
233 | hazard_curve | quantile curve (poe >= 0.15) for imt PGA
234 | hazard_curve | quantile curve (poe >= 0.5) for imt PGA
235 | hazard_curve | quantile curve (poe >= 0.85) for imt PGA
</pre>