update project structure to what nimble expects now

[SpotlightKid]

• Created src/ directory and moved msgpack4nim.nim and msgpack4nim dir into it.
• Added srcDir = "src" to msgpack4nim.nimble.
• Changed module imports in examples and tests:
  • Separated stdlib imports and msgpack4nim imports.
  • Used std/ prefix for stdlib modules.
  • Sorted module names.
• Added config.nims to examples/ and tests/ to set source path and output dir for examples and test binaries.
• Removed installFiles = @["msgpack4nim.nim"] from msgpack4nim.nimble.
• Added build/ dir to git ignore file.
• Changed tests/test_json.nim to use staticRead() to read JSON and msgpack test input files, so that test_json binary has run-time access to their contents regardless of location and doesn't need to be saved in tests directory.
• Removed obsolete msgpack.org.md file.
• Set minimum Nim version to 1.6.0 in nimble file.

[SpotlightKid]

PR branched from #76 to avoid conflict when that PR is merged.