Notes in subject

in notes model:

``` after_save :bimonthly_average ```  is a callback that updates the average bimonthly note.

``` bimonthly_data  ``` get the notes of  two months, It given a date dictation.

in subjects.js : 

``` upDateNote ``` method that obtained the average updated through an ajax

``` setAverageView ```  updates the average in the show subjects by sending a ajax that returns the current average.