Feature: export keys

"Tools" page, on the left.

**Should not be pulled without review!** Possible issues:

* exported to window, not file (accesible via DOM?..);
* exported in `<address>,<secret exponent>` format - might be hard to import;
* stealth keys not exported;
* iterating over addresses to get keys is (probably) sub-optimal;
* unit test is empty.

Making PR only since this seems to be a much-wanted feature in light of #164.