Added functions to generate seemingly random colours from numerical values.

The colours are chosen by hashing the input value, and then taking
3 bytes from the hash to use as colour components.
Moved hex fraction code into a new NumberUtils class.
Moved colour palette generation code into own classes.
The palette can now be specified when building the TMS tiles.

If not specified, defaults are chosen based on the domain of the data.
Using old palettes when none is specified for backwards compatibility.
Made NumberUtils.toHex work for negative numbers too.
Added documentation for colour palettes.
Added documentation for NumberUtils.
Fixed colour interpolation for rainbow palettes.
Added the ability to choose a palette when running a query.

Previously, there were hard-coded palettes for categorical and
continuous bands. Now there are a number of named palettes available
which can be chosen when running the map tile preview service.

Merge branch 'feature-hashcolours' into akka-workers