Support copying individual profiles or a whole Store

[andrewwhitehead]

Fixes #18

Also adds methods to get and set the default profile (opened when none is specified).

[TimoGlastra]

These changes seem very useful!

Need to make sure we also add these changes to the JS wrapper, but the changes surface of API seems quite small.

One question: Something in noticed with profile is that if I open a session for a profile that does not exist, it will not throw an error until a record is actually inserted of fetched from that session. This means I have to check whether a profile exists before opening, which is possible by calling createProfile. Or we have to fetch something to see if it works. I see some changes to the implementation of profiles, does this PR affect that behvaiour in any way?

What we do with normal wallets is that we try to open them, and if it fails we create them. This allows us to open it directly in all cases except the first time. While now we have to do a check every time that will fail everytime except the first time. It would be useful if opening a session would fail if the profile does not exist yet.

If this PR does not affect this at all, I can also open a separate issue for it

[andrewwhitehead]

@TimoGlastra Thanks for opening the issue. This PR doesn't change that behaviour (although it adds some profile-related methods) but it's a good one to fix for 0.3.