Timestamp() is wrong if np.datetime64 < 1678-01-01
The following should work, but since it doesn't should raise an error...

In [116]: pd.Timestamp(np.datetime64('1677-01-01'),'D')
Out[116]: Timestamp('2261-07-22 23:34:33.709551616', tz=None)

In [117]: pd.Timestamp(np.datetime64('1678-01-01'),'D')
Out[117]: Timestamp('1678-01-01 00:00:00', tz=None)

In [121]: pd.**version**
Out[121]: '0.11.1.dev-06cd915'

In [125]: sys.version
Out[125]: '3.3.2 (default, May 21 2013, 15:40:45) \n[GCC 4.8.0 20130502 (prerelease)]'
