virtualenv: ignore any venv directories

Problem: Sphinx will automatically traverse into the venv directory
causing errors.
See also: sphinx-doc/sphinx#1470

Solution: exclude the venv directory from both sphinx and git

.gitignore

@@ -1 +1,2 @@
-_build
+_build
+venv

conf.py

@@ -65,7 +65,7 @@
 # List of patterns, relative to source directory, that match files and
 # directories to ignore when looking for source files.
 # This pattern also affects html_static_path and html_extra_path.
-exclude_patterns = ['_build', 'Thumbs.db', '.DS_Store']
+exclude_patterns = ['_build', 'Thumbs.db', '.DS_Store', 'venv']
 
 master_doc = 'index'
 source_suffix = '.rst'