Stringpath binder

Related to https://github.com/playframework/Play20/pull/482

The first commit is the same as : https://github.com/playframework/Play20/pull/684

I added sub-types of `DynamicPart` (in the routes-compiler subproject):  
* `StringDynamicPart`
* `StringPathDynamicPart`
* `RegexDynamicPart`

I created the type `StringPath` which represent a path.