Makefile compatible with OS X

The Makefile now detects whether OS X (Darwin) is being used, and changes the
extension suffix accordingly from .so to .dylib. On current Apple hardware,
i386 architecture is no longer applicable, so it is removed from the build.

Note: two tests fail on OS X: testSave and testResize. Output:

```
.E...............[pcl::PLYReader] /var/folders/bm/j5txvfpx3qj4x7k5dc6hz9_r0000gn/T/tmpkzT9mLpcl-test/foo.ply:2: parse error
[pcl::PLYReader::read] problem parsing header!
[pcl::PLYReader::read] problem parsing header!
E...........
======================================================================
ERROR: testResize (test.TestExceptions)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/joris/Developer/project-patty/python-pcl/tests/test.py", line 230, in testResize
    self.assertRaises(MemoryError, self.p.resize, -1)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/unittest/case.py", line 475, in assertRaises
    callableObj(*args, **kwargs)
  File "pcl/_pcl.pyx", line 231, in pcl._pcl.PointCloud.resize (pcl/_pcl.cpp:4936)
    self.thisptr().resize(x)
RuntimeError: vector

======================================================================
ERROR: testSave (test.TestSave)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/joris/Developer/project-patty/python-pcl/tests/test.py", line 183, in testSave
    p = pcl.load(d)
  File "/Users/joris/Developer/project-patty/python-pcl/pcl/__init__.py", line 22, in load
    % (path, format))
IOError: error while loading pointcloud from '/var/folders/bm/j5txvfpx3qj4x7k5dc6hz9_r0000gn/T/tmpkzT9mLpcl-test/foo.ply' (format='ply')
```
uname -s tests the operating system. If this is equal to Darwin (= Mac OS X), then the dynamic library extension should not be .so but .dylib. The ARCHFLAGS variable is used by setup.py, and it determines which architecture to build for. This disables a warning that the i386 architecture could not be built: other libraries on OS X do not have their i386 architecture compiled so the binary cannot not be linked.=