IE <= 10 support.

[dettier]

This PR fixes #215 and returns IE <=10 support.
I turned on loose mode and added transform-proto-to-assign transform.
This is pretty safe to do for this library as it doesn't rely on that specs that are loosened.
As a side-effect bundle size went down 12%.

PS. If you extend ReactLeaflet classes in IE10 you still need to manualy "inherit" static properties:

// for IE9 & IE10 support
static contextTypes = BaseClass.contextTypes;
static childContextTypes = BaseClass.childContextTypes;

[PaulLeCam]

Thanks!