update to README and Makefile to support both Python 2 and 3

Makefile

@@ -1,7 +1,10 @@
 
-venv:
+venv-py2:
 	virtualenv venv.garminexport
 
+venv-py3:
+	python3 -m venv venv.garminexport
+
 init:
 	pip install -r requirements.txt
 

README.md

@@ -19,8 +19,10 @@ Library Import section below for more details.
 Prerequisites
 =============
 The instructions below for running the program (or importing the module)
-assumes that you have [Python 2.7](https://www.python.org/download/releases/2.7/),
-[pip](http://pip.readthedocs.org/en/latest/installing.html), and [virtualenv](http://virtualenv.readthedocs.org/en/latest/virtualenv.html#installation) installed.
+assumes that you have Python 2.7 or Python 3+,
+[pip](http://pip.readthedocs.org/en/latest/installing.html), and
+[virtualenv](http://virtualenv.readthedocs.org/en/latest/virtualenv.html#installation)
+(not required with Python 3) installed.
 
 It also assumes that you have registered an account at
 [Garmin Connect](http://connect.garmin.com/).
@@ -31,7 +33,14 @@ Getting started
 Create and activate a new virtual environment to create an isolated development
 environment (that contains the required dependencies and nothing else).
 
+    # using Python 2
     virtualenv venv.garminexport
+
+	# using Python 3
+	python -m venv venv.garminexport
+
+Activate the virtual environment
+
     . venv.garminexport/bin/activate
 
 Install the required dependencies in this virtual environment:

setup.py

@@ -22,4 +22,5 @@ setup(name="Garmin Connect activity exporter",
           'Natural Language :: English',
           'License :: OSI Approved :: Apache Software License',
           'Programming Language :: Python :: 2.7',
+          'Programming Language :: Python :: 3.5+',
       ])