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.

[MabezDev]

Very sorry about the delay in getting to this, I didn't have many spare cycles for this project over the summer.

With my one review comment, I'd accept this PR - but I'm curious as to how @avsaase's builder idea might work out.

[MabezDev]

Thanks!