Update dictionary!

[abolfazlaghdaee]

Hi friends, I wanna update dictionary file and add three informative and useful methods with their examples.

get()

for getting value of a key in dictionary

update()

for updating a dictionary with other dictionary

and
setdefault()

get or update a dictionary

Also I can add other methods but I think these are very important, so if you think same, please let me send PR and commit.
thanks