Fixed *nix build

I had to make some adjustments to build the wallet on Linux.

1: The designation of int64_t in statisticspage doesn't compile cleanly on Linux, as it requires a long long. I changed volume in statisticspage.cpp to qlonglong as it is typedef'd as int64 on Windows and long long on Linux. Nice quick fix.

2: The src/obj folder didn't exist, so I created it and put a little .gitignore hack in place to ensure it stays there

3: The src/leveldb/build_detect_unix file needs the executable bit on Linux to compile with the normal command sequence. I've enabled that.