Morphology functions

Hi guys! This PR just adds the `remove_small_connected_components` function to `skimage.morphology`. I've included tests and I've modified the `plot_coins_segmentation.py` example to use that instead of doing it manually.

I wasn't able to build the docs on my machine to test that modification though... I just did `cd doc; make`, and got the following error message:

```
 ~/projects/skimage/doc (morphology-functions)$ make
python tools/build_modref_templates.py
WARNING: Empty - skimage._shared
WARNING: Empty - skimage.scripts
Exception AttributeError: "'UmfpackContext' object has no attribute '_symbolic'" in <bound method UmfpackContext.__del__ of <scipy.sparse.linalg.dsolve.umfpack.umfpack.UmfpackContext object at 0x105109890>> ignored
Could not import PyQt4 -- skimage.viewer not available.
Could not import matplotlib -- skimage.viewer not available.
Could not import PyQt4 -- skimage.viewer not available.
Could not import PyQt4 -- skimage.viewer not available.
Could not import PyQt4 -- skimage.viewer not available.
WARNING: Empty - skimage.viewer.plugins
outdir:  
20 files written
Build API docs...done.
Generated ./coverage_table.txt from ./coverage.csv.
sphinx-build -b html -d build/doctrees   source build/html
Running Sphinx v1.1.3
/Users/nuneziglesiasj/anaconda/envs/skimdev/lib/python2.7/site-packages/matplotlib/__init__.py:1005: UserWarning:  This call to matplotlib.use() has no effect
because the the backend has already been chosen;
matplotlib.use() must be called *before* pylab, matplotlib.pyplot,
or matplotlib.backends is imported for the first time.

  warnings.warn(_use_error_msg)
loading pickled environment... not yet created
loading intersphinx inventory from http://scikit-learn.org/stable/objects.inv...
loading intersphinx inventory from http://docs.scipy.org/doc/scipy/reference/objects.inv...
loading intersphinx inventory from http://docs.scipy.org/doc/numpy/objects.inv...
loading intersphinx inventory from http://docs.python.org/2.7/objects.inv...

Exception occurred:
  File "<string>", line 7, in <module>
ImportError: cannot import name hough_circle
The full traceback has been saved in /var/folders/25/dnnk9t55369c3s4g4_3zdq4h0000gn/T/sphinx-err-dYpSfu.log, if you want to report the issue to the developers.
Please also report this if it was a user error, so that a better error message can be provided next time.
Either send bugs to the mailing list at <http://groups.google.com/group/sphinx-dev/>,
or report them in the tracker at <http://bitbucket.org/birkenfeld/sphinx/issues/>. Thanks!
make: *** [html] Error 1
```

It's the first time I try to build the docs, so any pointers are appreciated!