Added gaussian and erf functions.

[blas-ko]

Added gaussian and erf functions to the one variable expansions.
As these functions are not analytical, convergence is not tested.

An extra method gaussian(μ,σ,order) is included to automatically create a gaussian function with a given average (μ)and standard deviation (σ).

[lbenet]

Thanks for the contribution! Tests are failing.

[coveralls]

Coverage increased (+0.06%) to 98.703% when pulling 1d86fc7 on blas-ko:stat_functions into f897639 on JuliaDiff:master.

[blas-ko]

Test pass now. Maybe you can now check this @lbenet