Add a passwordInput widget

[cdevienne]

Description

Implements a PasswordInput widget.

I basically copied TextInput, removing its "chips" attribute and adding the "show" attribute. Adding "chips" back may be better, I would welcome feedback.

Pull request solving issue #55.

Checklist for new Widget

• [x] Checked out the Material design specification of the widget
• [x] Added constructors, Widgets Type and Style Type in src/Internal/[Your Widget].elm
• [x] Added styles in src/Internal/Material/[Your Widget].elm
• [ ] In src/Widget.Material.elm:
  • [x] Linked each style to its representative in src/Internal/Material/[Your Widget].elm
  • [ ] Added Documentation
• [ ] In src/Widget.elm:
  • [x] Added a copy of the Widget Type
  • [x] Added a copy of the Style Type
  • [x] Linked each constructor to its representative in src/Internal/[You Widget].elm
  • [ ] Replaced the Widget Type in the type signiture of each constructor with its definition
  • [ ] Added Documentation with a small example
  • [ ] run elm-verify-examples && elm-test to test the example

Optional:

• [x] Added an Example in example/src/Example/[Your Widget].elm

[Orasund]

thanks for your help