Emit warnings when deprecated elements are found in migration steps

[gggeek]

It would be a sensible strategy to start warning end users which keep using elements that will be dropped at some point in time in the future...
We probably have to find a good way for emitting the warnings though (via logger? to stderr?)

[gggeek]

Depends most likely on #201

[gggeek]

I'd go for having warnings emitted to both Sf log and stdout (the later of course only when migrations are run from the cli)

[gggeek]

Wondering if this is actually useful, given that:

• the next 'major' version of this (aka TanoConsulting/eZMigrationBundle2) will have a vastly different, incompatible yml migration definition dsl
• there are atm no plans to evolve this 'branch' to new versions which actually drop support for deprecated yml syntax...