Add ability to import games, sketches from external locations

[flaki]

Add ability to create sessions from external sources (rather than from those pre-installed on the server):

• Load a game/sketch from local filesystem
  This could be a single .ino file, a directory (on browsers that support uploading whole directories), or a zipped/tgz-d archive of files (which are automatically expanded after uploading).

• Load a game/sketch from a URL
  Similar to the above option, but sketches are downloaded from the URL and imported in similar fashion.
  This could make distributing games much easier, as a direct link could be added to any forum post like:

  http://clouduboy.slsw.hu/import?u=http://my.games/arduboy/coolstuff.zip
  

  Clicking the link takes the user into a new session automatically initialized with that sketch.
  For people who just want to play (and already have the Flasher installed on their system), they just have to click the "flash" button, but remixing/exploring/editing the source is just as easy from this on.

• Load a game/sketch directly from GIT (GitHub, Gist, etc...)
  Very similar to the above (github itself can be used to host game zips by linking to the release zip, or master zip), but a direct GIT access would open more possibilities for tighter integration in the future (e.g.: saving modifications to the files by saving back to the gist/committing them to a branch on GH and pushing the changes).