Create lex.l

Section 3.5
Update lex.l
Section 3.5
Section 3.5

Exercises from section 3.5
Section 3.5

Exercises from section 3.5
Section 3.5

Exercises from section 3.5
Rename lex.l to ..lex.l
Rename src/..lex.l to ch03/3.5/src/lex.l
Exercise 3.5.1
Exercise 3.5.4
Update lex2.l
Exercise 3.5.4
Exercise 3.5.4
Exercise 3.5.5
Update lex3.l
Update lex3.l
Update lex.l
Merge pull request #30 from dxhj/master

Add some source file for exercise 3.5