Move clock init to free func, update examples

[CBJamo]

Fixes #29

I feel like there should be a way to handle this that doesn't leave the footgun of forgetting the sd_init function, but I'm not sure how to do it.

[avsaase]

One idea is to use a builder pattern where you first need to create an uninitialized SD card struct, call init on that struct and then you get the regular SD card struct.