Various plotting fixes

[philippjfr]

A number of fixes to plotting in particular for the bokeh backend.

The fixes include:

• Empty datasources cause issues in 0.11.1, now setting the glyph invisible instead
• Datetime axes limits were being inferred incorrectly on OverlayPlots
• Fixed some outdated method signatures and unreferenced variables
• Fixed a bug with selection_specs in Columns.select that caused issues in plotting
• Fixed a bug in widget templating resulting in a unicode string with the u prefix being templated (causing JS syntax errors)

[jlstevens]

I've had a look through and all the fixes look good. Thankfully this is more a collection of small, localized fixes than a refactor! If it is ready, I'm happy to merge once the tests pass.

[philippjfr]

As you say just a bunch of small changes and tests are passing. Ready to merge.

[jlstevens]

Merged.