More rational Scales in composer and support for nautical miles on them

This is a little effort to make QGIS more usable for people related to the sea and marine scientists in particular, but also to provide a more rationale graphical scale system for the composer.

Scales in previous versions look somehow strange, because number of automatic units are sort of random. I propose to make more rationale scales. For instance, an automatic scale will be always be sort of "decimal" multiples, like 10, 20, 30or 25, 50, 75 or 5, 10, 15; The maginitude order of the quantity of the unit displayed in the scale is proportional to the width of the map but also is always multiple of ten.

I've tried to follow the schema provided in the previous code, making it possible to use multiples of the base units (meters and kilometers, for instance). also for imperial units; because I'm a metrical system user (except for nautical miles), I don't know if it's practical to use feet and miles, so please take a view on it, maybe it's better to use feet/yards/miles. In the case of Nautical miles, I don't know about practical uses of submultiples, but the same schema could be implemented.

Best regards,

Jorge Tornero