plainbox: add the decorators module

This patch adds the new module, decorators, where over time we'll
collect various utility decorators. Currently this module contains one
decorator ``@raises`` that can be used to annotate a function (or a
method) that it raises given exceptions.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
"automatic merge of lp:~zkrynicki/checkbox/decorators/ by tarmac [r=sylvain-pineau][bug=][author=zkrynicki]"
checkbox-ng: mock away dbus if MOCK_DBUS=yes

This somewhat crazy patch makes 'import dbus' and work well enough so
that dbus_ex and thus checkbox service "works" in testing. There are no
real tests for any of the dbus code so this, at least, allows us to test
checkbox-ng.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
.travis.yml: test checkbox-ng, checkbox-support, providers

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>