Add missing property index_range to 1D plots

[nicolasap-dm]

Adds a index_range property, synonym of index_mapper.range, to Base1DPlot. This corresponds to a similarly defined property in Base2DPlot, and Plot._handle_range_changed() expects this attribute to exist in order to work properly.

+ new assertions for regression testing.

Closes #490

[nicolasap-dm]

All python 3.6 CI builds are failing at the setup stage with

 OkonomiyakiError("Unsupported PKG-INFO metadata format: u'2.1'",)

[corranwebster]

A couple of minor nits.

Also there isn't a direct test of setting the index_range trait, only a second-order test of changing the index range of a Plot. It probably makes sense to add a setting test to the test case, and possibly a test that when you change the index_mapper that the index_range fires a trait change and is updated as expected (whether you do this depends on how much you trust the traits internals :) ).

[nicolasap-dm]

I've addressed the comments.

test that when you change the index_mapper that the index_range fires a trait change

I didn't have a module-level listener on index_range so I hooked one up to test, I hope that's canon

I made a drive-by style fix: all test functions were called "*scatter_" even for non-scatter plot modules. Since it may be off-topic, I committed it separately (cdd3cc7) to be able to revert it.

[nicolasap-dm]

Thanks, I didn't know about the mixin. I've made the changes.

[corranwebster]

LGTM