Improved coverage configuration

By providing a .coveragerc file with all default settings, users only
have to execute "coverage run ./runtests.py" without the need to specify
all the possible flags. The same applies to "coverage html" and
"coverage xml".

tests/.coveragerc

 [run]
-omit = */django/contrib/*/tests*,*/django/utils/autoreload.py
+branch = True
+omit =
+    */django/utils/autoreload.py
+parallel = True
+source = django
 
 [report]
 ignore_errors = True
+omit =
+    */django/conf/locale/*
+    */tests/*
 
 [html]
 directory = coverage_html