Include import of __future__ features division, absolute_imports and
print_function everywhere. Don't add unicode_literals yet for it is
harder to convert.
Goal is smoothing the transition to python 3.
Previously, an empty argument was treated as "not an argument at all". Now,
every function call always has at least one argument -- i.e., %foo{} is a
function call whose only argument is "" -- and %foo{,bar} is valid syntax.
This was causing a problem with situation where }} would have semantic meaning
other than escaping a }. Specifically, %func{%func{arg}} contains a }} but
should not escape the }. $} seems to cover this situation. However, ${ is not
permitted as an escape sequence because it looks like the beginning of a symbol
(variable reference) like ${foo}. This is OK because { can be used anywhere as a
literal.
