Material design: Text fields

<paper-input> is a single-line text field with Material Design styling.

<paper-input always-float-label label="Floating label"></paper-input>
<paper-input label="username">
  <iron-icon icon="mail" slot="prefix"></iron-icon>
  <div slot="suffix">@email.com</div>
</paper-input>

• Distribution is now done with slots, so things have changed because of that (before: foo, now foo; same for input, prefix and suffix)
• Since type extensions are not available, any use of <input is="iron-input"> has been replaced by <iron-input><input></iron-input>. See iron-input for more details about that change
• prevent-invalid-input and allowed-pattern had to be always used together; deleted prevent-invalid-input, so that only allowed-pattern is needed