Python: Add numpy as install requirement

[asfimport]

We natively link against NumPy, so it is no good idea to leave it out of the requirements list.

Reporter: Uwe Korn / @xhochy
Assignee: Uwe Korn / @xhochy

_{Note: This issue was originally created as ARROW-188. Please see the migration documentation for further details.}

[asfimport]

Uwe Korn / @xhochy:
PR: #69

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 69
#69