Tag GaussianMixtures v0.0.4

Changes from 0.0.3 to 0.0.4:
 - storing covariance matrices now as chol(inv(Σ))
 - computational speed improvements
 - better support for different datatypes, i.e., Float32 for GMM storage and data representation
 - improved type stability
 - import / export to Mixture Models
 - some restructuring
 - type conversions
 - weening off MAT dependence for i/o
 - some cosmetic changes in the code
 - better computation of memory requirement for full covariance training
 - bug fixes, issue https://github.com/davidavdav/GaussianMixtures.jl/issues/5

I hope that this is the basis for a more major release v0.1 soon.