BUG: Employee's age was not being computed.

Added a function inside the Person class that watches for changes
to the date of birth and computes the age of the employee
Merge pull request #168 from ericdill/employee_tutorial

BUG: Employee's age was not being computed.
DOC: Typo
Merge pull request #169 from ericdill/doc

DOC: Typo
DOC: Fixing typos in docs due to boring meeting
Merge pull request #173 from ericdill/doc

DOC: Typo fix