generate a URL

example::

    >>> str(URL(a='a', c='c', f='f', args=['x', 'y', 'z'],
    ...     vars={'p':1, 'q':2}, anchor='1'))
    '/a/c/f/x/y/z?p=1&q=2#1'

    >>> str(URL(a='a', c='c', f='f', args=['x', 'y', 'z'],
    ...     vars={'p':(1,3), 'q':2}, anchor='1'))
    '/a/c/f/x/y/z?p=1&p=3&q=2#1'

    >>> str(URL(a='a', c='c', f='f', args=['x', 'y', 'z'],
    ...     vars={'p':(3,1), 'q':2}, anchor='1'))
    '/a/c/f/x/y/z?p=3&p=1&q=2#1'

    >>> str(URL(a='a', c='c', f='f', anchor='1+2'))
    '/a/c/f#1%2B2'

    >>> str(URL(a='a', c='c', f='f', args=['x', 'y', 'z'],
    ...     vars={'p':(1,3), 'q':2}, anchor='1', hmac_key='key'))
    '/a/c/f/x/y/z?p=1&p=3&q=2&_signature=5d06bb8a4a6093dd325da2ee591c35c61afbd3c6#1'

generates a url '/a/c/f' corresponding to application a, controller c
and function f. If r=request is passed, a, c, f are set, respectively,
to r.application, r.controller, r.function.

The more typical usage is:

URL(r=request, f='index') that generates a url for the index function
within the present application and controller.

:param a: application (default to current if r is given)
:param c: controller (default to current if r is given)
:param f: function (default to current if r is given)
:param r: request (optional)
:param args: any arguments (optional)
:param vars: any variables (optional)
:param anchor: anchorname, without # (optional)
:param hmac_key: key to use when generating hmac signature (optional)
:param hash_vars: which of the vars to include in our hmac signature
    True (default) - hash all vars, False - hash none of the vars,
    iterable - hash only the included vars ['key1','key2']
:param scheme: URI scheme (True, 'http' or 'https', etc); forces absolute URL (optional)
:param host: string to force absolute URL with host (True means http_host)
:param port: optional port number (forces absolute URL)

:raises SyntaxError: when no application, controller or function is
    available
:raises SyntaxError: when a CRLF is found in the generated url

helper to encode the provided (binary) data into base64.