Release memory in cvode, kinsol and idasol.

This should address #24. The memory allocated by the library is released in the convenience functions `cvode`, `kinsol` and `idasol`. It basically follows the code posted by @rs1909 in #24. I couldn't figure out how to properly call the Julia function `CVodeFree`, which takes a pointer to a pointer (`Ptr{Ptr{Void}}`), so I used `ccall` instead.

@rs1909: Could you please check if this closes the leaks?