Add a "relayfee" field to getinfo.

This shows the minimum relay fee for non-free transactions in btc/kb.

The armory developers requested this so that they can prevent users from
 creating transactions that not even their local bitcoind will relay.

This also slightly reorders the getinfo output so that the fee related
 lines are grouped and changes the help text to reflect that the units
 are btc/kb.
Merge pull request #3868 from gmaxwell/minfeeinfo

Add a "relayfee" field to getinfo.