Fixed sign flip warnings for unsigned integers.

Detected by Visual Studio 2010.
Fixed floating point conversion warnings.
Boost: (Hopefully) Fixed linkage problems on Windows.

When using threads, one also has to link with chrono, date_time, and serialization.
Some of these components are not available in older Boost versions. What a mess.
Merge branch 'master' of github.com:viennacl/viennacl-dev