examples: enclose each example in a code block and clean up on errors

Signed-off-by: Greg Farnum <greg@inktank.com>
examples: add comment about the failings of Rados::pool_create()

Signed-off-by: Greg Farnum <greg@inktank.com>
examples: add comment with more details about using async IO

Signed-off-by: Greg Farnum <greg@inktank.com>
examples: write a basic Makefile instead of providing compiler instructions

Signed-off-by: Greg Farnum <greg@inktank.com>