gesdd, major refactoring of math headers, moved #factorize_lu to Ruby
source from C++.
Added gesdd specs
Updated history
Added NMatrix#gesdd and NMatrix#gesvd
Added #supershape for shape of slice parent
Updated N[] shortcut to NMatrix[]

Following John Prince's suggestion, I modified the N[] shortcut to use
NMatrix[] instead.

close 118
Updated spec for inline constructor
Updated other specs with NMatrix[]

Ops, almost forgot them.
Re-added N[] shortcut as an alias to NMatrix[]