Check swig is installed

Display an error message is swig is not installed.

I hope it will be clearer than the default error message:
error: command 'swig' failed: No such file or directory

setup.py

@@ -33,6 +33,7 @@
 
 from setuptools import setup, Extension
 from setuptools.command.build_py import build_py
+from distutils.spawn import find_executable
 
 
 platform_include_dirs = []
@@ -64,6 +65,11 @@
     except:
         platform_include_dirs = ['/usr/include/PCSC', '/usr/local/include/PCSC']
 
+if find_executable("swig") is None:
+    print("Install swig and try again")
+    print("")
+    sys.exit(1)
+
 VERSION_INFO = (2, 0, 7, 0)
 VERSION_STR = '%i.%i.%i' % VERSION_INFO[:3]
 VERSION_ALT = '%i,%01i,%01i,%04i' % VERSION_INFO