Add precompiled headers to improve build speed

This adds a new first build stage "phase1" which consists
of precompiling `stdlib.hpp` in src/lib as a general header
that is then force -included into all .cpp compilations.

This severely reduces i/o seek times for the standard library
and large-ish parts of boost and should in turn drastically reduce
HYRISE compile time.
Merge pull request #97 from bastih/feature/precompiled_stdlib_header

Add precompiled headers to improve build speed