Move yaml/yaml.h to yaml.h and merge version.c to api.c.

include/Makefile.am

include/yaml.h

include/yaml/yaml.h

include/yaml/yaml_error.h

include/yaml/yaml_version.h

src/Makefile.am

src/api.c

src/reader.c

src/scanner.c

src/version.c

tests/test-reader.c

tests/test-version.c