Add base for chapter 7
Add introduction about interruptions
Add link to chapter 7
Add keyboard example
Change interruptions to interrupts
Add infos about Programmable interrupt controler
Add table of common IRQs
Spelling fixes.
Merge pull request #22 from tcort/spelling

Spelling fixes.
Install zip by default,in order to run `make build`.
Merge pull request #27 from mikulely/patch-1

Install zip by default,in order to run `make build`.
Add qemu documentation
Spelling fixes.

Spelling fixes in Chapter 2.
README.md: Correcting punctuation
Some changes in Chapter 3
Merge pull request #32 from escalant3/chapter3-fixes

Some changes in Chapter 3
Merge pull request #31 from andrewSC/master

README.md: Correcting punctuation
Merge pull request #30 from Xumeiquer/master

Spelling fixes.
Typo: access -> accessed
Typo: base->lucid32

the box name 'base' is different from 'lucid32' in Vagrantfile, which causes download box twice, one time for 'vagrant add box' another time for 'vagrant up'