Timestamp does not parse the 'Z' zone designator for UTC
Pandas 0.14.1 does parse UTC 'Z' designator:

``` python
In [1]: import pandas as pd;pd.__version__
Out[1]: '0.14.1'
In [2]: pd.Timestamp("2014-11-10 11:00Z")
Out[2]: Timestamp('2014-11-10 11:00:00+0000', tz='UTC')
```

Beginning with pandas 0.15.0, it seems to be broken:

``` python
In [1]: import pandas as pd;pd.__version__
Out[1]: '0.15.0'
In [2]: pd.Timestamp("2014-11-10 11:00Z")
Out[2]: Timestamp('2014-11-10 11:00:00')
```
