allow introducer to provide default encoding parameters (k-of-n)

[warner]

For small+centrally-managed grids, the admin of the introducer should be able to recommend the values for k and n (as in k-of-n encoding). The value of 'n' should be set to be about the maximum number of peers in the network, since there's no point in producing more shares than there are peers (unless you expect more peers to arrive in the future, and we need to write more code to take advantage of new peers without reencoding anyways). The value of 'k' should be set to achieve whatever reliability-vs-expansion goals you have in mind.

For small networks, we're thinking that 3-of-10 seems like a good idea.