Refactor directory structure of Excalibur and separate repositories

[kamranayub]

I propose cleaning up the root directory structure of Excalibur because it's starting to get unwieldy.

• [x] Clean up dir structure
• [ ] Create sample game repository
• [x] Create docs repository [*]
• [x] Remove un-needed files (makefile?)
• [x] Improve contributing doc to walk through build process
• [x] Update VS2013 solution to reflect new changes

  Proposal:

- src
  - tools (nuget)
  - engine (ts)
  - specs (tests)
  - sublime.proj
  - vs2013.sln
  - others.csproj
- dist (distributed files)
- assets
- docs [*]
- package.json
- bower.json
- GruntFile.js
- etc.

I also propose the "sample-game" be renamed to "dev-sandbox" since that's more appropriate and a separate repository be created for real sample games with exalibur set as a git submodule, e.g. excalibur-samples.

For the Windows 8 sample, that should be moved to the samples repository and maybe switched to use another sample game like the shoot'em up.

[*] It might also be useful to separate docs into excalibur-docs but that's up for debate due to the website generation.

[eonarheim]

Moving create sample game repository to separate issue #248