Remove the fmt.Fprint fallback when encoding command arguments

[garyburd]

Change argument encoding to encode simple values using strconv and return an error for other values.

• The fmt.Fprint encoding is not useful for structured values like []int32.
• There is more than one useful encoding for structured values (json, msgpack, gob). The application should pick which encoding is used.

[garyburd]

• Consider using TextMarshaler (http://golang.org/s/go12encoding) as a fallback for structured values.
• Encoding and decoding values is something that every application needs to do. Consider updating Scan and Args to help with structured values.

[the42]

Unable to provide help, this would be a very helpful change. Switching from HMSET / AddFlat to manually JSON encode nested structs now.

[albrow]

I support using json as a fallback.

• Json enconding/decoding has been a feature of go for a long time, requires no extra dependencies, and is available for use now.
• It works for nearly anything you could throw at it.
• Lua scripting in redis can make use of the cjson library to parse json, whereas other formats would be unreadable on the lua scripting side. (EDIT: there's a msgpack library included as well.)
• Json is a human readable format, making it easier to debug.

@garyburd would you consider accepting a pull request if I put something together?

[garyburd]

@stephenalexbrowne I want to support arbitrary encoding formats with whatever mechanism is defined. JSON has a number of benefits, but the encoding is not ideal for all uses. Also, I want a corresponding feature for decoding values.

Let's discuss at https://groups.google.com/forum/#!forum/redigo

[albrow]

I started a thread at https://groups.google.com/forum/#!topic/redigo/ffrOEowXd4k

[garyburd]

New related issue: #46