human readable printing of uncaught exceptions

related to issue #84 

option 'humanReadableUnhandledException' enables nicer printing of the error stack in case of a unhandled exception.

with options is false (default):
```
2014-12-17T09:22:49.509Z - error: uncaughtException: my error date=Wed Dec 17 2014 10:22:49 GMT+0100 (CET), pid=23482, uid=1000, gid=1000, cwd=/home/samz/avuba/winston, execPath=/home/samz/.nvm/v0.10.31/bin/node, version=v0.10.31, argv=[node, /home/samz/avuba/winston/wintest], rss=13991936, heapTotal=9293056, heapUsed=3159968, loadavg=[0.44482421875, 0.50146484375, 0.42724609375], uptime=178087.771779633, trace=[column=7, file=/home/samz/avuba/winston/wintest, function=, line=21, method=null, native=false, column=26, file=module.js, function=Module._compile, line=456, method=_compile, native=false, column=10, file=module.js, function=Object.Module._extensions..js, line=474, method=Module._extensions..js, native=false, column=32, file=module.js, function=Module.load, line=356, method=load, native=false, column=12, file=module.js, function=Function.Module._load, line=312, method=Module._load, native=false, column=10, file=module.js, function=Function.Module.runMain, line=497, method=Module.runMain, native=false, column=16, file=node.js, function=startup, line=119, method=null, native=false, column=3, file=node.js, function=null, line=906, method=null, native=false], stack=[Error: my error,     at Object.<anonymous> (/home/samz/avuba/winston/wintest:21:7),     at Module._compile (module.js:456:26),     at Object.Module._extensions..js (module.js:474:10),     at Module.load (module.js:356:32),     at Function.Module._load (module.js:312:12),     at Function.Module.runMain (module.js:497:10),     at startup (node.js:119:16),     at node.js:906:3]
```

when true:
```
2014-12-17T09:22:54.048Z - error: uncaughtException: my error date=Wed Dec 17 2014 10:22:54 GMT+0100 (CET), pid=23498, uid=1000, gid=1000, cwd=/home/samz/avuba/winston, execPath=/home/samz/.nvm/v0.10.31/bin/node, version=v0.10.31, argv=[node, /home/samz/avuba/winston/wintest], rss=13991936, heapTotal=9293056, heapUsed=3160264, loadavg=[0.5693359375, 0.5263671875, 0.435546875], uptime=178092.310348735
Error: my error
,    at Object.<anonymous> (/home/samz/avuba/winston/wintest:21:7)
,    at Module._compile (module.js:456:26)
,    at Object.Module._extensions..js (module.js:474:10)
,    at Module.load (module.js:356:32)
,    at Function.Module._load (module.js:312:12)
,    at Function.Module.runMain (module.js:497:10)
,    at startup (node.js:119:16)
,    at node.js:906:3

```