[Tagging] Heliport tagging

[Daniel Koć]

I was trying to use helicopter icon for rendering helipads on default 
style, but it looks like it was not needed here and instead it could be 
good to be used for heliports. However they are currently unrecognizable 
from plane airports:

https://github.com/gravitystorm/openstreetmap-carto/pull/1806#issuecomment-137401140

This looks like a simple case for me: we don't use aeroway=heliport 
scheme at all, so we could just start now (probably leaving 
aeroway=aerodrome for planes and mixed plane/helicopter airports).

We have currently about 6k uses with value "heliport", so given that 
some of them are just in different languages, it might be at most - 
let's say- 5k of them in our database:

http://taginfo.openstreetmap.org/search?q=heliport#values

Comments?

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