Add get_duration method for Note object (#146)

* Add pycharm files to gitignore

* Add duration property to Note

@Property is not used - project uses get_X convention

* Add docstring for get_duration

.gitignore

@@ -77,3 +77,6 @@ Temporary Items
 Session.vim
 .netrwhist
 *~
+
+### pycharm ###
+.idea

pretty_midi/containers.py

@@ -27,6 +27,10 @@ def __init__(self, velocity, pitch, start, end):
         self.start = start
         self.end = end
 
+    def get_duration(self):
+        """Get the duration of the note in seconds."""
+        return self.end - self.start
+
     def __repr__(self):
         return 'Note(start={:f}, end={:f}, pitch={}, velocity={})'.format(
             self.start, self.end, self.pitch, self.velocity)