Implement component for adding todos to todo-list

[hehug7]

This implies creating functionality for adding new todos to the list. We probably want a modal window which pops up when you press a button, where you can add todos. To begin with, we want to support a simple todo which will be more advanced in a later issue. This includes the add-button.

[hehug7]

This will also include tests and css for the form and button for adding todos in addition to documentation of functions.

[torjusti]

Fixed in #15.