Add program that finds the checksum of a file (#5)
Add program that finds the checksum of a file (#5)

Add a simple utility that prints the checksum of a file in a base 16 format, the one we are using to compare the checksum of different files in our current mock utilities. It uses a buffer to avoid loading large files into memory.

By @sabinadayanova and @mariaprat 
googleinterns/executable-mocks,googleinterns/executable-mocks