getVersion: add utility to get version of buildbot or plugins

This will be usefull for out of tree plugins

Signed-off-by: Pierre Tardy <pierre.tardy@intel.com>
Merge tardyp/buildbot:getVersion (PR #1185)
add a note about one-line-per-sentence style
re-formatted the file according to one-line-per-sentence style
slightly change the structure of manual/customization
Merge sa2ajj/buildbot:docs-customization (PR #1186)