Add rounding modes to normal floating point arithmetic
Add Float32 rounding mode tests
Integrate the new rounding modes with BigFloats

This changes the old rounding mode sketch to a more intuitive one,
and using types instead of constants seems to be more logical.
Export RoundAwayZero too (even though it only works for BigFloats)
Stop hardcoding the `fenv.h` constants

As suggested by Viral in https://github.com/JuliaLang/julia/pull/3149
Fix bug while parsing fenv_constants.h
Renaming of the rounding modes
Fixes post-rebase
Remove fenv_constants.jl, as it is auto-generated
Documentation for the rounding modes (manual + stdlib)

It also includes some updates to some related sections about
BigFloats.
Fix issue with fenv.h and Ubuntu 12.04

If the values from fenv.h aren't found by the C preprocessor, it uses
the default ones for x86/x64
Merge pull request #3149 from andrioni/floatrounding

Ability to change rounding mode for all floats (cf. #2976)