http tracker server: error on missing params
better test error messages
http tracker: support multiple info_hash scrapes
move common encode/decode fns to common.js
client: make tracker errors into 'warning' events

trackers failing to respond or giving non-200 http status codes is not
a fatal error
Support scrape without info_hash specified (Fix #28)
only /announce and /scrape are valid endpoints
add single scrape, multi scrape, all scrape tests
1.9.0