Do not create a temporary array when obj is not specified in create_array

This is a solution for avoiding the creation of temporaries when creating a regular Array object when specifying  the atom and shape only.  Fixes #337.

Memory consumption is now similar to h5py, and by running the examples in [this post](https://groups.google.com/forum/#!topic/h5py/_8-bTZuwbdE) (but with n = 50000 to make things a bit faster), we get:

with h5py:
```
$ rm test.h5; time python benchmark-h5py.py
Creating HDF5 file...
0 1 2 3 4 5 6 7 8 9 Elapsed: 0.02 s
Bandwidth: 77.6 MB/s

real    0m4.284s
user    0m3.854s
sys     0m0.420s
```

with PyTables:
```
$ rm test.h5; time python benchmark-tables.py
Creating HDF5 file...
0 1 2 3 4 5 6 7 8 9 Elapsed: 0.01 s
Bandwidth: 225.7 MB/s

real    0m3.825s
user    0m3.283s
sys     0m0.535s
```

which is pretty much faster than h5py (don't know exactly why).