Implemented Quantity-aware wrappers for assert_allclose and linspace

As discussed in https://github.com/astropy/astropy/issues/1274, it would be great to have Quantity-aware wrappers to some Numpy functions. We don't have to monkey-patch these, we can just provide them in a separate namespace. This PR can be used e.g. as:

```
In [2]: from astropy import units as u

In [3]: from astropy.units import numpy_wrappers as unp

In [4]: unp.linspace(1 * u.cm, 1. * u.m, 100)
Out[4]: 
<Quantity [   1.,   2.,   3.,   4.,   5.,   6.,   7.,   8.,   9.,  10.,
             11.,  12.,  13.,  14.,  15.,  16.,  17.,  18.,  19.,  20.,
             21.,  22.,  23.,  24.,  25.,  26.,  27.,  28.,  29.,  30.,
             31.,  32.,  33.,  34.,  35.,  36.,  37.,  38.,  39.,  40.,
             41.,  42.,  43.,  44.,  45.,  46.,  47.,  48.,  49.,  50.,
             51.,  52.,  53.,  54.,  55.,  56.,  57.,  58.,  59.,  60.,
             61.,  62.,  63.,  64.,  65.,  66.,  67.,  68.,  69.,  70.,
             71.,  72.,  73.,  74.,  75.,  76.,  77.,  78.,  79.,  80.,
             81.,  82.,  83.,  84.,  85.,  86.,  87.,  88.,  89.,  90.,
             91.,  92.,  93.,  94.,  95.,  96.,  97.,  98.,  99., 100.] cm>
```

I'd like to include a few functions like this in 0.4 if there are no objections.