Implements negative index in NMatrix. Fixes #174

Example:
```
a = N[1, 2, 3, 4]
a[-1] # output => 4
a[-3] # output => 2
a[0..-1] # output => [1, 2, 3, 4]
a[0..-2] # output => [1, 2, 3]
a[-7] #output => RangeError
```