Replace Modal's default true prop: dismissable

[interactivellama]

JSX props should not be true by default.

My suggestion is to deprecate dismissable and remove it from the propTypes and use the following.

/**
 * If true, Modals cannot be dismissed by clicking on the close icon or pressing esc key.
 */
disableClose: React.PropTypes.bool,
``

This should first be warned with a console error with dismissable is used.

This should not be a breaking change.

[aswinshenoy]

I wish to work on this, I believe I can do this in a day.