#17: Add support of dict unpacking

[nvbn]

It was failing before changes, like described in #17.
After changes:

In [1]: import ast
In [2]: from astunparse import unparse
In [3]: print(unparse(ast.parse('{a:b, **c}')))
{
    a: b,
    **c,
}

[nvbn]

https://travis-ci.org/simonpercivall/astunparse/jobs/230782095#L587 failed because in Python 3.6.1 FormattedValue.format_spec is JoinedStr instead of Str. It's fixed in #19

Tests failed on travis-ci with python 3.2 with unrelated error - https://travis-ci.org/simonpercivall/astunparse/jobs/230798076

[graingert]

@nvbn if you rebase on #21 you can get the tests passing.