curl_json plugin: Use the "number" callback of libyajl.

The "integer" callback only works with "long"s, which are 32bit on
x86 and other 32bit architectures. The "number" callback gets the raw
string for us to parse ourselves – honoring the data source type in the
process.

The "integer" and "double" callbacks have been removed, since they are not
used if the "number" callback is present.