more generic QR: generalize the impl of the Indexable trait

This allows the implementation of householder reflection without relying
on knowledge of DVec. This required a new member in the Indexable trait:
the shape() function, which returns the maximum index available.
generic implementation of QR decomposition

But static matrices can't use it yet, they need to implement the
Row/Col slicing traits.
QR factorization for fixed size matrices.

The ColSlice implementation for fixed size matrices returns a DVec,
while this is probably not optimal performance-wise, the dynamic nature
of the result makes this necessary. Using a data type presenting the
ImmutableVector trait would solve this, but it looks like a non-trivial
change.
fix typo
new_identity and housholder matrix available under na::
Merge remote-tracking branch 'origin/master' into genericQR
update to latest rust
QR decomposition mentionned in README
update to the latest rust: FloatMath for math functions (sin/exp/...)

Also removed a bunch of duplicate trait usages
Merge pull request #9 from vbarrielle/genericQR

Generic QR factorization