Fix a few deprecations on Julia 0.7

[martinholters]

Remaining deprecation warnings at the moment are all due to the moval of mean and middle to Statistics. I'll fix those once JuliaLang/Compat.jl#583 has landed. Whether as part of this PR or in another one is a matter of how fast we get the Compat. I'd like to tag a version with the new unwrap soon, but wanted to fix those deprecations where it was easy. So if the Statistics thing takes its time, I'll tag before, despite the remaining deprecations.

[martinholters]

esprit: Test Failed at /home/travis/build/JuliaDSP/DSP.jl/test/estimation.jl:20
  Expression: isapprox(frequencies', frequencies_estimated; atol=0.01)
   Evaluated: isapprox([1000.0; 1250.0], [999.958, 1249.95]; atol=0.01)

in one of the CI runs. We should probably loosen the bound here. EDIT: Restarted that job.