Comparison of write operations depending on buffer size.

Chapter 13, exercise 1.
Explain flushing buffers from user and kernel space.

Chapter 13, exercise 3.
Difference in write order depending on a file type.

Chapter 13, exercise 4.
Add an implementation of the tail(1) utility.
Fix typo in comments.
Prefer `strtol` over `atol`.
Merge pull request #5 from rmascarenhas/chap13

#13 - File I/O Buffering