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]"