React 0.12 support alternate (temp _currentElement.key solution)

Provide React 0.12 support in all react-bootstrap components.
This PR uses the private _currentElement.key value to avoid making API changes to react-bootstrap during this transition.

WARNING: POTENTIAL BREAKING CHANGE:

For some reason, number key values of child elements are cast to strings.  This can cause failures when parent elements perform comparisons with those key values.  For example, the below example will not work because, at runtime, the child keys get converted to strings, but the parent defaultActiveKey does not `(1 === '1') -> false`.

```
    <TabbedArea defaultActiveKey={1} animation={false}>
      <TabPane key={1} tab="Tab 1">TabPane 1 content</TabPane>
      <TabPane key={2} tab="Tab 2">TabPane 2 content</TabPane>
    </TabbedArea>
```

This version will work:
```
    <TabbedArea defaultActiveKey='1' animation={false}>
      <TabPane key='1' tab="Tab 1">TabPane 1 content</TabPane>
      <TabPane key='2' tab="Tab 2">TabPane 2 content</TabPane>
    </TabbedArea>
```

1 potential solution to this problem would be to change the relevant comparisons within react-bootstrap to use `==` or `!=` instead of `===` or `!==`

@stevoland 
@pieterv