Rework map and map!

[timholy]

There are several things that are a bit complicated/confusing about
map and map!:

• the init keyword also affects whether f runs (controlling the initial
  value is not orthogonal to execution, which spells trouble for
  functions with side effects)
• in contrast with Base.map!, map!(f, dest::Observable, args..)
  does not eagerly update dest---that doesn't happen until one of
  args updates
• map always yields an Observable{Any}, somewhat in contradiction
  to the behavior of Base.map (see map does not infere type #34)

This redesigns map and map! to address these issues (fixes #34).
It is a breaking change, although I've put in a depwarn for the case
where init was supplied. (The non-deprecated changes are so fundamental
that this may not be useful, however.)

Now:

• map always runs f on initial creation
• map! typically runs f on the initial call, but it can be
  suppressed with an update=false keyword
• map creates an Observable{T} with specific T, but users can
  control output eltype by using map!(f, Observable{S}(), args...)
  instead.

Using the new API, the old map behavior can be obtained with

    map!(f, Observable{Any}(), args...)

or with an init it's

    map!(f, Observable{Any}(init), args...; update=false)