add `log_level` to base app aliases

'log_level' alias to 'Application.log_level' is added to base_alises defined
in IPython.core.application, which is inherited by most IPython applications.

closes gh-541

IPython/core/application.py

@@ -54,6 +54,7 b' from IPython.utils.traitlets import List, Unicode, Type, Bool, Dict'
 base_aliases = dict(
     profile='BaseIPythonApplication.profile',
     ipython_dir='BaseIPythonApplication.ipython_dir',
+    log_level='Application.log_level',
 )
 
 base_flags = dict(