Update widget to 0.6.9.1, bump version number to 0.18
Merge branch 'master' into file-group

Conflicts:
	tests/test_file.py
Add requirements.txt for development purposes
Add ``docs/_build`` into .gitignore
Add Sphinx config and Makefile
Add documentation structure
Add api reference documentation
Add installation section
Add quickstart section
Add short description of CLI section
Update quickstart.rst by more appropriate texts
Add intro into index.rst
Add deprecated bits section into doc structure
Add description of django widget settings
Remove django widget api settings
Add content of Model Fields section
Add File.cropped, File.resized into deprecated.rst
Add File.keep into deprecated.rst
Merge pull request #37 from uploadcare/sphinx-doc

Sphinx doc
Replace load_file_from_cache by File.construct_from