Changes in vex::multivector interface

There are no more non-owning multivectors in VexCL. vex::multivector
class has only two template parameters now: type and number of
components.

vex::tie() now returns vex::expression_tuple instead of non-owning
multivector. This allows to tie vectors of different types or even
writable expressions (e.g. slices) together.
Merge pull request #98 from ddemidov/vexcl-0.8-multivector-interface-changed

Changes in vex::multivector interface