Infrastructure improvements (documentation tests & performance)

This is adding two new features (that I've been working on every now and then) to the testing infrastructure. This should be fairly easy to review, because it pretty much only adds stuff (without even changing much in Deedle).

The only Deedle change is in the convertor for R provider - I added handling of `RDotNet.EvaluationException`, because otherwise the documentation sample calling `R.austres.Value` was failing (I suspect this is an R.NET change).

## Tools for measuring performance

The idea is that each time we have a new release, we can copy the binaries to `tests\Performance\builds` (with some small config). The `performance.fsx` script (which really just runs `Deedle.PerfTest.dll` that is in the `PerformanceTools.sln` solution) will then build the code in the `Deedle.PerfTests` project and run it against all the different versions. The script then produces two charts (showing absolute and relative speedup) I used here: http://tomasp.net/blog/2014/deedle-v1/index.html

## Documentation tests

I also added (or, got to work) unit tests that automatically look at all `fsx` files in the documentation and make sure that there are no compilation errors and no evaluation errors. While doing this, I also fixed a few doc issues :-)