Added Gaussian window function.

I'm using Aquila in a personal project based on a MATLAB prototype, and I needed a Gaussian window function to do some blurring, similar to MATLAB's gausswin() (see http://www.mathworks.com/help/signal/ref/gausswin.html). This pull adds that function based on the formula at http://en.wikipedia.org/wiki/Window_function#Gaussian_window, as well as a (admittedly-basic) test.

As a demonstration, here's the MATLAB output:
```matlab
>> gausswin(10)
ans =
    0.0439
    0.1510
    0.3812
    0.7066
    0.9622
    0.9622
    0.7066
    0.3812
    0.1510
    0.0439
```
and here's the output of the Aquila::GaussianWindow() function with the same size and their default standard deviation (they use α = 1/σ as input, default α is 2.5, so σ = 0.4).
```C++
Aquila::GaussianWindow gaussian(10, 0.4)
0.0439369
0.151007
0.381171
0.706648
0.962154
0.962154
0.706648
0.381171
0.151007
0.0439369
```