Fix NonNegQuadReg and init_nndsvd!

[david-vicente]

• Previously we couldn't use NonNegQuadReg because it was not declared a subtype of Regularizer, as GLRM methods' signatures require it to be.
• The evaluate method for NonNegQuadReg used a Julia function that is no longer available in current Julia versions.
• prox! for NonNegQuadReg was also not working for some reason, so I used the fix from #100 as a baseline to solve this. I don't know if it is correct though.
• added NonNegQuadReg to README.
• init_nndsvd! was returning matrices with non contiguous columns, my fix simply passes those matrices through Matrix() constructor before returning them.