Open serialization API

`EasyRdf_Serialiser_Turtle` class has encoding methods, which are useful in other scenarios (such as SPARQL-queries generation). Unfortunately, all of it's internal mechanics is `protected` while some of the methods could be `public static`

* `serialiseResource()` could be splitted in 2 methods. one to get string-representation and another to actually encode it (braces, escaping)
* `quotedString()` is useful as is. just make it `public static`
* `serialiseObject()` would be useful as `public static` method of its own but extracting part about literals into separate method(s) would make sense too

would you accept this as a patch? :)