Updated examples to use pyplot for plotting instead of pylab

Addresses #3109 .

Used `git grep "import pylab as pl" | while IFS=':' read file line; do sed -e 's:import pylab as pl:import matplotlib.pyplot as plt:' -e 's:\bpl\b:plt:g' -i $file; done;` to perform the replace operation. 

I performed a basic test by running all the files changed. The following ran into some errors (not all related to the above change) and hence have been excluded from the commit :-
```
applications/plot_hmm_stock_analysis.py
applications/plot_model_complexity_influence.py
applications/plot_out_of_core_classification.py
applications/plot_stock_market.py
decomposition/plot_ica_vs_pca.py
document_classification_20newsgroups.py
ensemble/plot_forest_iris.py
linear_model/plot_sparse_recovery.py
tree/plot_tree_regression_multioutput.py
```
I think the error in `applications/plot_hmm_stock_analysis.py` is related to #2646