Fix some comment typos.
Update install command to also install utilities.

This commit updates the install command under the Installation section in
README.md to include all subdirectories thereby including the utilities
such as addblock and btcctl.
Updated getinfo to return new RelayFee and use btcjson.InfoResult
Improve non-standard tx error message.

This commit removes the word "only" from the non-standard transaction
error message when the number of items on the stack does not match the
number of expected items.

This was suggested by @mbelshe.
Change rate limit debug messages to trace level.