area and MetaFree fixes

[knuesel]

Some fixes for area following discussion in #121

• area(::AbstractVector{AbstractPoint}}) is now documented
• In particular, the doc says that a signed value is returned in the 2D case
• Fixes for custom indices (OffsetArrays is added as a test dependency)
• The 3D case now accepts vectors of any AbstractPoint{3} type (the 2D case already did)

There's also a broken area test for a vector of PointMeta{3}. I couldn't make it work because it needs zero(typeof(PointMeta(0,0,0; a=1)) and it's not clear how to implement that: How should we make the zero value of the metadata?

The PR includes a fix for a related bug: MetaFree(typeof(PointMeta(0,0,0; a=1)) used to return 3. Now it returns Point{3,Int64}, I think that's what was intended? (the fix isn't enough to make area work but I included it in the PR since it took me a while to figure out).

I also added Julia 1.3 in the CI test matrix, to avoid a repeat of #123 😊

[SimonDanisch]

Thank you! :)