Base component __repr__ logic is wrong.

[rmarren1]

Base component __repr__ is broken. It assumed that _prop_names[0] is always children, which is not true.

This breaks some things, for example if a component does not have children and thus its first prop is id, it will print the id as the first positional argument rather than a proper keyword argument. This is mostly relevant for dash-html-components.

For example:

Python 2.7.15rc1 (default, Nov 12 2018, 14:31:15) 
[GCC 7.3.0] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import dash_core_components as dcc
>>> dcc.RadioItems(id='hello') 
RadioItems('hello')
>>> import dash_html_components as html
>>> html.Div(id='hello')
Div(id='hello')

[rmarren1]

will fix in upcoming PR for #311