Support the passing of objects returned by eval() straight to set()

[Kevin-Jin]

Rserve.create().eval() accepts a callback function with signature (error, robj). Rserve.create().set() should be able to accept such a robj (without any transformations) so that set() effectively is the complement to eval(). This would enable applications to pass objects that require attributes, such as data frames and matrices, to R by constructing a valid SEXP robj.

set() should also accept all results of robj.json() as well, although the result in R may not be exactly the same as the what is generated by eval(). E.g. because of the type ambiguity of arrays that are all NA, set('x', eval(as.numeric(NA))) will save a logical vector to 'x'.

I'm working on a pull request for this now but I won't submit it until pull request #32 is either accepted or denied because there could be a lot of conflicts depending on the outcome.