Build PECOFF/DWARF dump_syms for MinGW

Substitute MapViewOfFile() for mmap()
Fix building client library and upload tools for MinGW

This is the rest of https://breakpad.appspot.com/548002/, brought up to date

Mainly fix include filenames to be lowercased for case-sensitive filesystems

Removed usage of stdext::checked_array_iterator, which is a MSVC-only feature.
first try at building MinGW client library and upload tools using gyp-generated Makefile
Add a README.md with notes on what is left to do