Pkg.clone: allow leading [^\/] for auto package name detection.

Closes #6141 and #6152.
replace some ? with ifelse

add hex(::Char, ...) so Char doesn't need abs()

add conversions from Char to both Signed and Unsigned
Merge branch 'master' of github.com:JuliaLang/julia
Windows CI using AppVeyor

New configuration file appveyor.yml
- Downloads and installs latest Julia binary, uses it for dependency libraries
- Also uses MSYS from bundled Git installation

Build script contrib/windows/msys_build.sh
- Downloads MinGW, LLVM, make, etc
- Sets up Make.user to use as many downloaded dependencies as possible
Use Cygwin, build both arches