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[]
Merge pull request #119 from agarie/update-shortcut

Aliased N[] shortcut to NMatrix[]