Minor enhancements

Get rid of NonfatalException (an exception is nonfatal iff it is
catched somewhere)

Foreport fixes for #26 and #30.

The current __sersion__ string is now kept in instaloader.py rather than
setup.py. This lets instaloader.__version__ always deliver the version
of the actually-loaded Instaloader module.

Minor changes to README.rst, error handling and which class methods are
public.

With these and the changes of the previous commit, we saved 31 lines of
code, indicating that it might be easier to understand and to maintain.

setup.py

@@ -1,14 +1,25 @@
 #!/usr/bin/env python3
 
+import re
 import sys
 from setuptools import setup
 
+
+def get_version():
+    with open('instaloader.py') as f:
+        for line in f:
+            m = re.match("__version__ = '(.*)'", line)
+            if m:
+                return m.group(1)
+    raise SystemExit("Could not find version string.")
+
+
 if sys.version_info < (3, 5):
     sys.exit('Instaloader requires Python >= 3.5.')
 
 setup(
     name='instaloader',
-    version='2.2.2',
+    version=get_version(),
     py_modules=['instaloader'],
     url='https://github.com/Thammus/instaloader',
     license='MIT',