Initial VSX commit
First time it compiles, but fails to pass the tests.
32-bit floats/ints, 64-bit doubles pass packetmath tests, complex 32/64-bit remaining
VSX port passes packetmath_[1-5] tests!
VSX supports vec_div, implement where appropriate (float/doubles)
prefetch are noops on VSX
fix compile error on big endian altivec
Merged eigen/eigen into default
prefetch are noops on VSX, actually disable the prefetch trait
Merged in kmargar/eigen (pull request #84)

Add VSX support
Many improvements in Divide&Conquer SVD:
- Fix many numerical issues, in particular regarding deflation.
- Add heavy debugging output to help track numerical issues (there are still fews)
- Make use of Eiegn's apply-inplane-rotation feature.
Bug 879: fix compilation of tri1=mat*tri2 by copying tri2 into a full temporary.
Bug 880: manually edit the DartConfiguration.tcl file to get it working with cmake 3.0.x
Bug 881: make SparseMatrixBase::isApprox(SparseMatrixBase) exploits sparse computations instead of converting the operands to dense matrices.