Some more API/Doc suggestions

[bobes]

1. The value of balance in response to account command is a string,
although it contains a number. The same goes for the value of
credits_cost in response to message_status command. Other numeric
parameters have number values. Although it's not a bug, returning all
numeric parameters with number values would improve usability.

2. The values of created_time and completed_time in response to
message_status command are strings but the value of timestamp in
response to receive command is number. All these parameters contain
numeric UNIX timestamps. It would be nice to have them all as numbers.

3. One of the examples for the receive command response has a string
value for unread, while the real responses contain number values.