Tag DataStructures v0.2.15, v0.3.0 (Feedback wanted before merge)

I wanted to get feedback before merging.  See the discussion [here](https://github.com/JuliaLang/DataStructures.jl/issues/41) and [here](https://github.com/JuliaLang/DataStructures.jl/commit/162249b625014952b066ba661eb258129991c94c).

This PR tags DataStructures v0.2.15 as the latest version of DataStructures that supports Julia v0.2, and v0.3.0 as the first version which only supports v0.3.

Within DataStructures, the change which prompted this was only the `@test_throws` macro changes, so the versions are identical except for testing changes right now.

DataStructures now has a `julia_v0.2` branch, which currently points at tag v0.2.15.  (`master` points at v0.3.0)

Future changes to DataStructures could be backported to the `julia_v0.2` branch and tagged consecutively from 0.2.15.  (@StefanKarpinski, for this to happen, does the tag need to be specified manually?)

If this hasn't been codified, I'd like (again) to suggest

1. setting the required version of julia to `0.3-` in REQUIRE
2. bumping the minor (or major) version number when tagging, and 
3. creating a `julia_v0.2` branch based on the previously tagged version

as the recommended method for packages adjusting to breaking changes in new versions of Julia.

Thoughts/comments?

Cc: @lindahua, @IainNZ, @StefanKarpinski