used plt.subplots everywhere in examples

converted all plots in examples to
```
f, ax = plt.subplots(nrows=*, ncols=*,  figsize=(*, *))
```
fixes Issue [#777](https://github.com/scikit-image/scikit-image/issues/777)

### To Do
* doc/examples/plot_glcm.py
* doc/examples/plot_local_otsu.py
* doc/examples/plot_phase_unwrap.py
* doc/examples/applications/plot_rank_filter.py

#### Issue with the above:
how to do the following with `plt.subplots()`
```
img1 = np.zeros(10, 10)
img2 =  img1 + 1
plt.figure(figsize=(8, 5))

plt.subplot(1, 2, 1)
plt.imshow(img1, cmap=plt.cm.gray)
plt.title('title1')
plt.colorbar(orientation='horizontal')
plt.axis('off')

plt.subplot(1, 2, 2)
plt.imshow(img2, cmap=plt.cm.gray)
plt.title('title2')
plt.colorbar(orientation='horizontal')
plt.axis('off')
```