Merge pull request #1 from GitbookIO/feature/cli

Initial gitbook binary
Add base display of exercise
Add content & code generation for exercise sections
Add base for full display of exercises
Generate a unique Id for each section
Filter out empty sections when generating pages
Clean output folder before generating
Add lex sub-command to gitbook binary

The lex command is useful for debugging files and seeing how they are
parsed
Set size of exercise code editor according to solution

Use the number of lines in solution to set editor’s size
Remove test.js
Improve page section design
Change summary data format

Simplify chapter objects, by making them identical to article objects
besides the .articles attribute
Add parse.navigation module

With tests
Add footer for navigation links
Improve spacing of navigation links in footer
Add navigation parsed in footer
Mark active chapter in summary
Improve title position with summary open in header
Fix title position for small screen
Add navigation tests for first page for  README of a chapter