Logging tweaks

[SimonWoolf]

Fixes #273. See #273 (comment) for explanation and justification of changes. Basic idea is to keep LOG_MAJOR for high-level, user-friendly state information. If the user wants to peek into the inner workings of the lib (eg see connection upgrading, individual transports, potentially see raw protocol messages, etc), they need LOG_MINOR or higher.

Console log with LOG_MAJOR for connecting, channel attach, and one token renewal cycle, before changes:

Ably: ConnectionManager.chooseTransport(): Transport recovery mode = recover; connectionKey = undefined; connectionSerial = undefined 
Ably: ConnectionManager.chooseTransport(): upgrading ... connectionKey = 1082b!cdOMLOUkZzqbyBtS-4bad1082b 
Ably: Transport.onProtocolMessage(): error; connectionKey = 1082b!cdOMLOUkZzqbyBtS-4bae1082b; err = {"message":"Key/token status changed (expire)","code":40142,"statusCode":401} 
Ably: ConnectionManager.chooseTransport():  
Ably: ConnectionManager.chooseTransport(): Transport recovery mode = resume; connectionKey = 1082b!cdOMLOUkZzqbyBtS-4bae1082b; connectionSerial = -1 
Ably: ConnectionManager.chooseTransport(): upgrading ... connectionKey = 1082b!cdOMLOUkZzqbyBtS-4baf1082b

After changes:

Ably: Connection state: connecting 
Ably: Connection state: connected 
Ably: Channel state for channel "channel": attaching 
Ably: Channel state for channel "channel": attached 
Ably: Connection state: disconnected; reason: Key/token status changed (expire), code: 40142 
Ably: Connection state: connecting 
Ably: Connection state: connected

[paddybyers]

lgtm

[SimonWoolf]

Merged as b39872e