Support non-string keys for LinkMaps

[quux00]

Currently, the ogonori LinkMap implementation - for both reading (deserializing) and writing (serialization) only support string keys. The map is of the form:

map[string]*oschema.OLink

However, according to the OrientDB schemaless serialization docs (http://orientdb.com/docs/last/orientdb.wiki/Record-Schemaless-Binary-Serialization.html), the key for a LinkMap is allowed to be:

STRING,SHORT,INTEGER,LONG,BYTE,DATE,DECIMAL,DATETIME,DATA,FLOAT,DOUBLE

Modify ogonori to support this. This will primarily involve changing obinary/binserde.go - the readLinkMap and writeLinkMap methods, and then updating the client.go functional test. Other parts may need to change as well.