Added support for supplying fixed ticks with labels

[philippjfr]

Finally adds ticking support with fixed labels to the bokeh backend, e.g.:

%%opts Curve [xticks=[(0, 'Foo'), (1, 'Bar'), (2, 'Baz')]]
hv.Curve([1,2,3])

Will be available when 0.12.6 is released, both the code and the tests are appropriately guarded to work only with bokeh version > 0.12.5.

[jlstevens]

Great! How about we warn and suggest upgrading bokeh if this ticker format is detected?

[jlstevens]

Looks good. Merging.