Support StaticArrays

This package is almost ready to be used straight up with StaticArrays.
Most variables are already written in a general way, there are just two
blockers:
The preallocated buffers `JJ` and `n_buffer`, aswell as the LMResults
type.

This commit fixes that, and almost allows usage of StaticArrays.
However, there's still
JuliaDiff/DiffResults.jl#25, but that can be
currently overcome by two typedefs.

src/levenberg_marquardt.jl

@@ -27,8 +27,8 @@ end
 
 struct LMResults{O,T,Tval,N}
     method::O
-    initial_x::Array{T,N}
-    minimizer::Array{T,N}
+    initial_x::AbstractArray{T,N}
+    minimizer::AbstractArray{T,N}
     minimum::Tval
     iterations::Int
     iteration_converged::Bool
@@ -146,8 +146,8 @@ function levenberg_marquardt(
     # Create buffers
     n = length(x)
     m = length(value(df))
-    JJ = Matrix{T}(undef, n, n)
-    n_buffer = Vector{T}(undef, n)
+    JJ = transpose(jacobian(df))*jacobian(df)
+    n_buffer = similar(x)
     Jdelta_buffer = similar(value(df))
 
     # and an alias for the jacobian