Update README.md and HACKING.md

Move HACKING to HACKING.md.
Add notes on audio in README.
Add notes on coding style, Git workflow, dependencies in HACKING.
gfx: Move SDL initialization to gfx_init(), gfx_deinit()
gfx: Add higher-level sprite drawing functions

Add sprite drawing functions for all sprite
types in gfx, and use these functions throughout
the source code.
gfx: Wrap underlying SDL rectangle drawing functions

Use gfx_draw_rect() and gfx_fill_rect() throughout
the source code.
gfx: Add frame drawing functions
gfx: Add screen resolution functions
sdl-video: Remove unused masked sprite cache parameter

This was previously used to locally cache masked sprites in viewport. This
is no longer used since the introduction of viewport landscape prerendering.
Fix bug where cursor could be freed more than once
Move sprite_t to dos_sprite_t
Add Travis CI configuration file
README: Add build status from Travis CI
Fix: include misc.h in gfx.h for uint definition
sdl-video: Move sprite_t conversion to gfx.c

Sprite caching is moved to gfx.c as well.
sdl-video: Move all palette use to gfx.c
sdl-video: Move sprite masking to gfx.c
gfx: Merge sprite caches into single hash table
sdl-video: Merge all sprite drawing functions into one
Abstracting graphics and data source

Set apart of basic and common functionality for graphics and data
source.