Add screenshot to front page

Also:
- Updated pythong_scripting.py example to show how to colorize text
- Updated contributing.md to show how to run code coverage in parallel using python-xdist and -nauto
- Updated contributing.md to talk more about looking at unit test results within the PR

Author: tleonhardt

CONTRIBUTING.md

@@ -274,7 +274,7 @@ where `4` should be replaced by the number of parallel threads you wish to run f
 Code coverage can be measured as follows:
 
 ```shell
-py.test --cov=cmd2 --cov-report term-missing --cov-report=html
+py.test -nauto --cov=cmd2 --cov-report=term-missing --cov-report=html
 ```
 
 Then use your web browser of choice to look at the results which are in `<cmd2>/htmlcov/index.html`.
@@ -390,6 +390,14 @@ how to do it.
 
 6.  Indicate what local testing you have done (e.g. what OS and version(s) of Python did you run the
     unit test suite with)
+    
+7.  Creating the PR causes our continuous integration (CI) systems to automatically run all of the
+    unit tests on all supported OSes and all supported versions of Python.  You should watch your PR
+    to make sure that all unit tests pass on Both TravisCI (Linux) and AppVeyor (Windows).  
+    
+8.  If any unit tests fail, you should look at the details and fix the failures.  You can then push
+    the fix to the same branch in your fork and the PR will automatically get updated and the CI system
+    will automatically run all of the unit tests again.
 
 
 ### How We Review and Merge Pull Requests

README.md

@@ -13,6 +13,8 @@ applications.  It provides a simple API which is an extension of Python's built-
 of cmd to make your life easier and eliminates much of the boilerplate code which would be necessary
 when using cmd.
 
+[![Screenshot](cmd2.png)](https://github.com/python-cmd2/cmd2/blob/master/cmd2.png)
+
 
 Main Features
 -------------

cmd2.png

@@ -36,7 +36,7 @@ def __init__(self):
     def _set_prompt(self):
         """Set prompt so it displays the current working directory."""
         self.cwd = os.getcwd()
-        self.prompt = '{!r} $ '.format(self.cwd)
+        self.prompt = self.colorize('{!r} $ '.format(self.cwd), 'cyan')
 
     def postcmd(self, stop, line):
         """Hook method executed just after a command dispatch is finished.