Add REQUIRE, trim exports, work on 0.4

[garborg]

Closes #13, closes #14.

[eytan]

As a heads up, a number of these changes are already in my PR, https://github.com/johnmyleswhite/StreamStats.jl/commit/eb7b894bd4b9b199be5d940d1652a8e437a3d85c

[garborg]

Thanks, @eytan. If you're planning to release that soon, certainly feel free to wait and incorporate the rest of the changes there.

If it might be a while, it would be good to tag a patchlevel release to squash the 0.4 bug and make the package work for users who don't already have Distributions installed.

[johnmyleswhite]

Should be handled by commit by @eytan.

[garborg]

Great!