Next-action

Added

- Next-action can now read multiple todo.txt files to select the next action from. For example:
`next-action --file todo.txt --file big-project-todo.txt`. Closes 35.

Changed

- Ignore tasks that have a start date in the future. Closes 34.
- Take creation date into account when determining the next action. Tasks created earlier take precedence. Closes 26.

Added

- Specify the number of next actions to show: `next-action --number 3`. Closes 7.
- Show all next actions: `next-action --all`. Closes 29.

Added

- Allow for limiting the next action to one from multiple projects: `next-action +DogHouse +PaintHouse`. Closes 27.
- Allow for limiting the next action to multiple contexts: `next-action work staffmeeting`. Closes 23.

Added

- Allow for limiting the next action to a specific project. Closes 6.

Changed

- Renamed Next-action's binary from `next_action` to `next-action` for consistency with the application and project
name.

Added

- Allow for limiting the next action to a specific context. Closes 5.
- Version number command line argument (`--version`) to display Next-action's version number.

Fixed

- Runing tests with "python setup.py test" would result in test failures. Fixes 15.
- Move development dependencies to requirements-dev.txt so they don't get installed when a user installs Next-action.
Fixes 14.
- Make Travis generate a wheel distribution in addition to the source distribution. Fixes 14.