Renamed format to reformat, (in "Rsips::Sips" module). Added specs

The 'format' method defined dynamically defined in "Rsips::Image" would clobber the 'format' method defined in "Rsips::Sips", so was unable to reformat images. 

**I have renamed format** (in the Rsips::Sips module) to "reformat" this avoids the conflict, although **it will break any existing code**. However a positive side-effect that the new method follows the convention of other methods in that module, those being…
* reset
* resample

I have also added some specs to the project, as every project should have some :-)