Get rid of 'ambiguous definition' warnings in base/floatfuncs.jl

Base library functions 'round', 'ceil', 'floor' and 'trunc' now can have second Integer argument (defined in base/floatfuncs.jl). This caused warnings about ambiguous definitions during loading of package Sims.
Merge pull request #3 from magistere/warnings

Get rid of 'ambiguous definition' warnings in base/floatfuncs.jl