Cast nvlength to int32

Fixes the following error on 32-bit systems:

ERROR: no method pointer_to_array(Ptr{Float64},(Int64,))

when calling Sundials.asarray

Tested by running the ida example on this configuration:

    $ uname -a
    Linux lab15-lenovo 3.2.0-67-generic-pae #101-Ubuntu SMP Tue Jul 15 18:04:54 UTC 2014 i686 i686     i386 GNU/Linux
    $ cat /etc/lsb-release 
    DISTRIB_ID=Ubuntu
    DISTRIB_RELEASE=12.04
    DISTRIB_CODENAME=precise
    DISTRIB_DESCRIPTION="Ubuntu 12.04.5 LTS"

And on this other one:

    $ uname -a
    Linux ignacio-laptop 3.13.0-33-generic #58-Ubuntu SMP Tue Jul 29 16:45:05 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux
    $ cat /etc/lsb-release
    DISTRIB_ID=Ubuntu
    DISTRIB_RELEASE=14.04
    DISTRIB_CODENAME=trusty
    DISTRIB_DESCRIPTION="Ubuntu 14.04.1 LTS"

All the examples work with your solution on 32 and 64 bits. Should I do another pull request or do you just change it?