[PYTHON] Refactor trainnig API to use callback

tests/python/testing.py

@@ -0,0 +1,22 @@
+# coding: utf-8
+
+import nose
+
+from xgboost.compat import SKLEARN_INSTALLED, PANDAS_INSTALLED
+
+
+def _skip_if_no_sklearn():
+    if not SKLEARN_INSTALLED:
+        raise nose.SkipTest()
+
+
+def _skip_if_no_pandas():
+    if not PANDAS_INSTALLED:
+        raise nose.SkipTest()
+
+
+def _skip_if_no_matplotlib():
+    try:
+        import matplotlib.pyplot as _     # noqa
+    except ImportError:
+        raise nose.SkipTest()