Adds capability to mark tests as slow

conftest.py

@@ -0,0 +1,5 @@
+import pytest
+
+def pytest_addoption(parser):
+    parser.addoption("--skip-slow", action="store_true",
+            help="skips slow tests")

xarray/tests/__init__.py

@@ -114,6 +114,12 @@ def requires_bottleneck(test):
     return test if has_bottleneck else pytest.mark.skip('requires bottleneck')(test)
 
 
+slow = pytest.mark.skipif(
+    pytest.config.getoption("--skip-slow"),
+    reason="set --skip-slow option to skip slow tests"
+)
+
+
 class TestCase(unittest.TestCase):
     if PY3:
         # Python 3 assertCountEqual is roughly equivalent to Python 2