Fix OnCell behavior for Quadratic{Flat}
Visual tests + fixes
First attempt at linear bc/extrap for B2
Scrap ExtendInner BC
Infrastructure for padding the equation system

The extra equations from the boundary conditions will not always allow
indexing as we did before, by e.g. doing cm_d = cp_d at the lower boundary
for some BC:s. In order to generalize better, we instead pad the equation
system with one line for each extra equation to allow more general BC specs.

This had the advantage of significantly simplifying some of the code (and,
even better, some of the concepts) required to implement new functionality,
so even though some of the underlying infrastructure is now more complicated
than before, new contributors will hopefully have a lower threshold to start
adding features.
Implement a few more BC:s + whitespace and test fixes
Fix some issues after rebasing
Free BC for Quadratic B-splines
Periodic BC/extrapolation
Export aux methods with info about interp type
Notebook with visualizations of behavior
Fix one (of several) problems with multidim interpol.

This fixes one of the bugs leading to the errors shown in [this comment][1],
but there's (at least) one remaining off-by-one error...

[1]: https://github.com/tlycken/Interpolations.jl/pull/9#issuecomment-68164696
Fix broken size implementation (resolves #14)
Fix broken multidimensional interpolation
Rename LinearBC -> Line (with typealias Natural)
Simple 2D tests
Run select tests from Grid.jl
Cleanup
Added 2D example to IJulia notebook
Disable Grid tests for now, since they're so brittle