Refactor a bit of sillyness in nom::Color & nom::Coords

- nom::Color
  * Remove nom::RGBA class and relocate methods to nom::Color and rename
methods: asSDLCOLOR to SDL; asRGB to pixel, asInt32 to RGB

- nom::Rect
  * Rename asSDLRect to SDL; this method needs to be implemented...

- nom::Coords
  * Introduce SDL method
Remove set_colorkey method call in Texture::load

* See note made in Image::set_colorkey
Add support for rendering Textures with clipping values set

* This allows us to efficiently render portions of a larger Texture in
memory, such as how we do with sprite sheets
Introduce several new methods

* New methods: width, height, pixels, pitch, bits_per_pixel,
pixel_format, pixel -- these are for SDL1 surface-level manipulations,
not generally for SDL2 Textures.
Add surface conversion for image surfaces

* This ought to be, in theory, more efficient...
Nothing to see here, move along...

* Fix spacing / tabbing