doc: basic documentation for the project
new strategy for the Python2/3 compatibility
refactor: all HTML related stuff is set in the same file.
refactor: simplification of the Widget class.
Changes for Python 3
Python 3 metaclasses
More Python 3 changes
.gitignore skips the documentation's build directory
some minor fixes and informations.
Removing of the old test infrastructure.
test: remove nose dependency.
Merge pull request #14 from svenstaro/master

Changes for Python 3
tests: HTMLAttrs
Travis CI configuration
use of six for py2/3 compatibility.
Reintegration of the last PR
refactor: simplification of the schema related classes.
refactor: simplification of the modifiers.
new modifiers
multiple fixes + reimplementation of the customize method in Widget