cep: merge CEPs in to trunk

This patch merges CEPs (aka Checkbox Enhancement Proposals) into trunk.
Those lived on separately for a while as lp:checkbox/cep but in
retrospective nobody knows about them and this should give them some
more exposure. In addition, the move allows new features to land a CEP
document along, making review of complex new features easier to make as
their specification can be seen alongside the patches that implement it.

Due to bazaar limitations in merging separate repositories together I've
discarded history entries (not that there were many) and just added
those files in directly.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
"automatic merge of lp:~zkrynicki/checkbox/cep/ by tarmac [r=sylvain-pineau][bug=][author=zkrynicki]"