Add a character counter to paper-input-decorator

Added a character counter option to paper-input and paper-input-decorator, to match the Material Design spec. The counter is only displayed if a `maxChars` value is passed.

Single-line input:
![normal](https://cloud.githubusercontent.com/assets/1369170/5117726/4e0d0c12-700d-11e4-8d0c-e0de4021df09.png)

Single-line input with error:
![error](https://cloud.githubusercontent.com/assets/1369170/5117728/4e0e2f3e-700d-11e4-9016-ea2fb27b4aea.png)

Multi-line input with error:
![textarea](https://cloud.githubusercontent.com/assets/1369170/5117727/4e0d4380-700d-11e4-814f-17b4c8957844.png)