Fix a BigFloat display error

If the default buffer wasn't enough, the function creates a bigger
buffer big enough to hold the string, but I wasn't accounting for
the null character.
Add eps(::Type(BigFloat))

The implementation is very direct, thanks to the availability of
nextfloat.
Add a conversion method from BigInt to FloatingPoint

This will be very useful in the future, as it solves #2930 for
BigInts (`float(x)` uses `convert(FloatingPoint, x)`), so
float(BigInt(1)im) now works.
Merge pull request #3059 from andrioni/eps

BigFloat string and eps, close #3056
probably fix #3055
generic fallbacks for the 4 stooges itrunc, iceil, ifloor, iround (part of #3040)
FloatingPoint->Int128 conversion (existing code mostly works for BigFloat too)
more BigFloat to int conversions
fix a couple other BigFloat issues
Merge pull request #3060 from andrioni/convert

Add a conversion method from BigInt to FloatingPoint
Merge branch 'master' of github.com:JuliaLang/julia

Conflicts:
	base/mpfr.jl
Change `using Test` to `using Base.Test` in the tutorial
Merge pull request #3063 from andrioni/patch-2

Change `using Test` to `using Base.Test` in the tutorial
Add Example.jl to the Travis section of the Packages chapter.
Update the travis-ci example for packages.
quick fix for #3064
Merge branch 'master' of github.com:JuliaLang/julia
Update section on contributing to core or base libraries in CONTRIBUTING.md
Merge branch 'master' of github.com:JuliaLang/julia