Add boundschecks to fix #35

[mattwigway]

Pretty much what the title says - for the three functions that take min and max values, confirm that both arrays are the same length as the RTree has dimensions. Add tests for same.

The MWE described in #35 now gives this output:

ERROR: DimensionMismatch: Minimum values must have same length as RTree dimensions
Stacktrace:
 [1] intersects(rtree::RTree, minvalues::Vector{Float64}, maxvalues::Vector{Float64})
   @ LibSpatialIndex ~/.julia/dev/LibSpatialIndex/src/LibSpatialIndex.jl:218
 [2] top-level scope
   @ REPL[9]:1

and if run at the REPL, returns you to the julia> prompt.