[MRG] DOC more complete coverage of API linking in examples

Examples in the documentation currently hyperlink to the API reference where a particular class or function is used. The current implementation has a few flaws, which this PR fixes:
* only plot examples currently have hyperlinks added (for example, [feature_selection_pipeline](http://scikit-learn.org/dev/auto_examples/feature_selection_pipeline.html))
* hyperlink determination currently involves ad-hoc regular expressions, rather than AST traversal
* it also runs the code and uses the resulting `locals` which, I think, is responsible for the spurious linking of `estimator` in [plot_learning_curve](http://scikit-learn.org/dev/auto_examples/plot_learning_curve.html)
* matching of hyperlink text was non-greedy, so that e.g. `plt` rather than `plt.xlabel` is linked in [plot_learning_curve](http://scikit-learn.org/dev/auto_examples/plot_learning_curve.html)
* etc.