Update \SNMP class signatures

[miken32]

Per the documentation for SNMP::get():

If objectId is a string, then SNMP::get() will return SNMP object as string. If objectId is a array, all requested SNMP objects will be returned as associative array of the SNMP object ids and their values.

I updated the return types for SNMP::get() and SNMP::getnext() to reflect this. I also updated the arguments to use their canonical (camel case) names and corrected a mistake in the ordering of two arguments to SNMP::walk().

One change I wasn't sure about: can arguments can accept complex expressions like return types do? The documentation for SNMP::set() says:

If objectId is string, both type and value must be string too. If objectId is array, value must be equal-sized array containing corresponding values, type may be either string (it's value will be used for all objectId-value pairs) or equal-sized array with per-OID value.

So I adjusted the argument types to reflect this logic but am not sure if this is ok or not.