Add a few convenient functions to create URLs to the Request type.

The three added functions allow an easier creation of URLs.  This can be
useful when, in the application, the developer needs to generate an
absolute URL (e.g.: for a redirect, to link to another resource, etc).
When constructing the URI base, simplify the lookup for the trailing /
Add tests for Request and change the type of parameters for
UriForWithParams.

The argument "parameters" was a hash of string, but since a query string
can contains multiple query with the same name, the new argumenent is
expected to be map of slices containing strings.

Some tests were added to validate the behavior of `UriBase`, `UriFor`
and `UriForWithParams`.
Merge pull request #19 from franckcuny/fc-uris

Helpers for creating URIs