v0.3.0 - API changes and aligned option groups

Backwards incompatible API changes

• option_groups decorator now takes the list of options as positional arguments *options;
• Group.section decorator now takes the list of sections as positional arguments *sections;
• The argument align_sections_help of Group (and group()) was renamed to align_sections;
• The argument sorted_ of GroupSection.__init__() argument was renamed to sorted.

Other changes

• Additional signature for option_group: you can pass the help argument
  as 2nd positional argument.
• Aligned option groups (option align_option_groups with default True).
• More refactoring and testing.