[api] Allow to set volume as a relative step count

[xvello]

Fixes #272 by allowing a client to increase the volume by 10 steps (or decrease it by 5 steps).

To be discussed:

• I don't accept both volume and step being present, but we could change this to give precedence to volume instead.
• As the player already has volume clamping in its volumeUp/volumeDown logic, I don't validate that step is in any range.
• The player could have a unique changeVolume(int steps) method instead of overloading volumeUp and volumeDown

# Valid requests
$ curl -X POST -d "step=10" localhost:24879/player/set-volume
$ curl -X POST -d "step=-5" localhost:24879/player/set-volume
$ curl -X POST -d "volume=30000" localhost:24879/player/set-volume

# Invalid input: both present
$ curl -X POST -d "step=15&volume=30000" localhost:24879/player/set-volume
{"name":"step","reason":"Cannot be passed alongside volume"}

# Invalid input on existing volume param
$ curl -X POST -d "volume=-30000" localhost:24879/player/set-volume
{"name":"volume","reason":"Must be >= 0 and <= 65536"}

# Invalid input on new step param
$ curl -X POST -d "step=0" localhost:24879/player/set-volume
{"name":"step","reason":"Must be non zero"}
$ curl -X POST -d "step=A" localhost:24879/player/set-volume
{"name":"step","reason":"Not an integer"}

[devgianlu]

• Good
• Good
• I am keeping the those methods for backward compatibility

I thought you wanted to bypass the step value and step up/down the volume by an arbitrary amount, but it's perfectly fine this way too.