Updated the readme to incorporate local installation of istanbul

[samhstn]

We wanted to do a local installation and had to look up the instructions online so we thought we should add them to the tutorial and we also added clarification on what the four metrics mean in the code coverage report

[nelsonic]

@shouston3 this is a great update! Thanks! If you don't mind, please change line 52 from

sudo npm install istanbul -g

to

npm install istanbul -g

Note: Stop using sudo to install node modules... http://stackoverflow.com/questions/16151018/npm-throws-error-without-sudo

then we can merge this in. :+1:

[nelsonic]

Thanks @shouston3 :+1: :rocket: