add twitter chapter
add various references
add skeleton chapters 1 and 3
add skeleton for chapters
adjust somethings in model
add taxonomy of categorization
add abstract of categorization
setup basic information
fix duplicated reference
setup year
add more 2 subsections
add more information about chapter 3
add tables for message codification
add more subsections
Merge branch 'add-references'
add math formulas
finish first step for categorization chapter
finish chapter 3
add chapter 4 - learning algorithms
Merge branch 'add-chapter-three'