Use Julia v0.6 syntax

[wsshin]

This PR incorporates Julia v0.6 syntax to the package. Now that StaticArrays uses Julia v0.6 (JuliaArrays/StaticArrays.jl#183), it seems reasonable to require v0.6 as the minimum Julia version.

Here is the summary of changes:

• abstract → abstract type ... end
• type → struct (instead of mutable struct, assuming the internal data of Object do not change in most cases)
• use of where ... for parametrization
• sumabs2 → sum(abs2, ...)
• dot syntax for min, max, sqrt, etc

[stevengj]

immutable etc still work in 0.6 with no warning. I don't want to switch to the new keywords until 0.6 is out for a while and most people have switched

[wsshin]

I see... That makes sense. Thanks!

[wsshin]

Wait... Actually I get the following deprecation warnings in 0.6:

WARNING: deprecated syntax "abstract Object{N}" at /Users/wsshin/.julia/v0.6/GeometryPrimitives/src/GeometryPrimitives.jl:6.
Use "abstract type Object{N} end" instead.

WARNING: deprecated syntax "inner constructor Ellipsoid(...) around /Users/wsshin/.julia/v0.6/GeometryPrimitives/src/ellipsoid.jl:8".
Use "Ellipsoid{N,D}(...) where {N,D}" instead.

and so on.

Also, I get

WARNING: sumabs2(x) is deprecated, use sum(abs2, x) instead.
...
WARNING: sumabs2(A, region) is deprecated, use sum(abs2, A, region) instead.
...
WARNING: min{T1 <: Real, T2 <: Real}(x::AbstractArray{T1}, y::AbstractArray{T2}) is deprecated, use min.(x, y) instead.
...
WARNING: max{T1 <: Real, T2 <: Real}(x::AbstractArray{T1}, y::AbstractArray{T2}) is deprecated, use max.(x, y) instead.

How about these warnings? If you want, I can create a PR to take care of them only.

[wsshin]

OK, I figured that the second set of warnings (about sumabs2 and min, max) can be eliminated without breaking compatibility with Julia v0.5.

However, the first set of warnings above (about abstract and inner constructors) will need to be just ignored for now, because fixing them will make the package incompatible with Julia v0.5. (Question: is this something that can be handled by Compat? If possible, it will be nice to eliminate these warnings as well.)

[stevengj]

The Compat package can be used to eliminate the other warnings without breaking backward compatibility

[wsshin]

Just made a new PR #2.