Added route requirements for the asset variables

This is needed to ensure the correct matching of routes. Otherwise, the
route for the combined asset /main.{locale}.js will be matched when accessing
the url for the leaf /main.en__1.js for instance, using the wrong value
for the variable.
It is also needed to avoid weird restrictions on the allowed values when
generating the route. The autogenerated requirement for the variable in
the combined url would forbid / and . (as the next delimiter char is a
dot), while the url for the leaf url would forbid / and _ for the last
variable. There is no reason to impose such weird restriction on possible
variable values.
On the other hand, the list of allowed values is known by the bundle (it
is required to allow dumping assets). So we can generate the proper
requirement to forbid other values in the url (which would generally break
the rendering by trying to access a non-existent file).
Merge pull request #251 from stof/asset_variables

Added route requirements for the asset variables