Add utcfromtimestamp method/function

cc @chdoig @cpcloud 

```Python
In [1]: from blaze import *

In [2]: d = Data([1, 2, 3])

In [3]: d.utcfromtimestamp
Out[3]: 
  _1_utcfromtimestamp
0 1970-01-01 00:00:01
1 1970-01-01 00:00:02
2 1970-01-01 00:00:03
```