Accept an options hash passed to Rubinius::FFI::Library#attach_function

This will make it easier for code targetting the FFI gem to use `Rubinius::FFI` when it is available by allowing keyword arguments to be passed to `Rubinius::FFI::Library#attach_function`.  It will resolve issue #3074.  An example of code using options passed to `attach_function` is here:
https://github.com/chuckremes/ffi-rzmq-core/blob/master/lib/ffi-rzmq-core/libzmq.rb#L62-L69

Currently, this patch accepts an options hash, but ignores all options.

The options used by the FFI gem are listed here, in the implementation:
https://github.com/ffi/ffi/blob/master/lib/ffi/library.rb#L214-L217

The `:blocking` option is a hint to the FFI gem that the C function may block and allows it to prepare MRI to release the GIL.  It seems like this option can be safely ignored in Rubinius.
The `:calling_convention` option tells the FFI gem what calling convention to use.  AFAIK, this is only to support windows API calls, and thus can safely be ignored in Rubinius.
The `:enums` and `:type_map` options are features that could be added to `Rubinius::FFI`, but in my opinion should be in a separate, more extensive patch.

I also cleaned up a few points of apparent cruft in the function implementation, including reindexing the argument names to make more sense and getting rid of a few assignments that were redundant or assigned to variables that were never used.

I also noticed that a `Pointer` is accepted as the `c_name`, so I documented that fact.  However, there are currently no specs for this.  I'll add some in a separate PR.