Not very userfriendly to allow overwriting previous frames

I realized (after doing it myself accidentally) that by 
allowing the user to set their own frame start and
storing things by frame number that we allow
users to accidentally overwrite previous frames without
any supervision.

I think it's better (and easier to future categorize 
capture events) to instead use timestamps for file
names. Since we can't go back in time without manually
changing the clock and the nature of this project
is taking a picture every *N*s we know that names
will be unique.

This also simplifies the surface area of the binary.