Add selector for flux units, preserved when flicking through transients.

Adds a 'flux_prefix' key in the HTTP query;
this is picked up in the `tableheader` render tag and also the new
flux_unit filter tag, to properly render the table headers and column
values respectively.

Also adds an include tag for rendering a flux-prefix selection
dropdown-button (this has the units_map added to its context,
in order to loop over the allowed options).

Finally, the prev/next button code was getting quite messy,
so I've moved it to a 'transient_paginator.html' sub-template.
These buttons now preserve flux_prefix query value, so
while flicking through transients in a given dataset, flux_prefix
choice is preserved.
Example of how to change default flux prefix in local_settings.py
Only build on Python 2.7
Merge pull request #25 from jdswinbank/flux-unit-selector

Flux unit selector