System exit if Cython modules are not imported

Added a condition that if cython modules are not imported system exits. Also removed functions printing warning and error msgs (were not useful)
Fix file paths
Minor correction in docstring of the wavread function
small type
change to take care of the N/2+1 issue
Added two sounds 101Sine and 200Sine
some small changes
changes to lecture slides
fixed a typo in slide
small changes
Merge pull request #1 from MTG/master

Prior to Exercise 1