WIP: Add unit tests for all code examples

TODO:
- [ ] Finish `test_fig_5()`
- [ ] Finish `test_fig_6()`

@astrofrog At the moment I'm stuck with `test_fig_5()` because I don't know how to write a Table in `ipac` ascii format:
```
======================================================================
ERROR: test_examples.test_fig_5
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/nose/case.py", line 197, in runTest
    self.test(*self.arg)
  File "/Users/deil/work/papers/astropy-v0.2-paper/examples/test_examples.py", line 117, in test_fig_5
    t.write('catalog.tbl', format='ascii', Writer=ascii.Ipac)
  File "/Users/deil/Library/Python/2.7/lib/python/site-packages/astropy-0.3.dev4471-py2.7-macosx-10.8-intel.egg/astropy/io/registry.py", line 224, in write
    writer(data, *args, **kwargs)
  File "/Users/deil/Library/Python/2.7/lib/python/site-packages/astropy-0.3.dev4471-py2.7-macosx-10.8-intel.egg/astropy/io/ascii/connect.py", line 25, in write_asciitable
    return write(table, filename, **kwargs)
  File "/Users/deil/Library/Python/2.7/lib/python/site-packages/astropy-0.3.dev4471-py2.7-macosx-10.8-intel.egg/astropy/io/ascii/ui.py", line 280, in write
    lines = writer.write(table)
  File "/Users/deil/Library/Python/2.7/lib/python/site-packages/astropy-0.3.dev4471-py2.7-macosx-10.8-intel.egg/astropy/io/ascii/ipac.py", line 91, in write
    raise NotImplementedError
NotImplementedError
```