Remove rake dependency

[kikito]

Rake was introduced as a "quick and dirty" commodity - Lua missions was inspired by Ruby Koans, so it was easy to just copy the Koans' rakefile and start from there.

Much as I like ruby, I think requiring it just to work with some Lua files is a bit weird. Ideally, if you already have Lua, you shouldn't need anything else to work with the lua-missions (appart from git, if you are making a pr)

The current rakefile has the following tasks:

$ rake -T
rake git:add           # add all the modifications to the git repository, including deletions
rake git:commit        # Commit with a message
rake git:push          # Push to origin
rake git:release       # regenerate missions & push to the git repository
rake missions:delete   # Delete the generated mission files
rake missions:gen      # Generate the Missions from the changed source files
rake missions:regen    # Generate the Missions from the source files from scratch
rake missions:run      # Trying to execute the missions
rake missions:run_src  # Execute the solved missions

• All the tasks in the git namespace (git:add, etc) can be ignored - most of them can be replaced by 1 or 2 simple git commands.
• missions:delete is only a "pre-task" used by mission:regen
• missions:gen is a nice-to-have, but isn't really needed (as long as you have missions:regen)
• missions:run does literally cd missions; lua missions.lua. And missions:run_src does something similar. I am quite tempted to just ignore them. If not, creating a lua script which executes them is fairly trivial.

So I think only missions:regen is needed. What I am thinking about is a lua script called script/generate.lua, which does what missions:regen currently does:

• Remove the missions folder and recreate it.
• For each file inside src:
  • Replace __(<anything>) by __
  • Delete the lines between -- begin skip and -- end skip, both included

This is troublesome for several reasons:

• Getting a list of files inside a folder isn't possible in vanilla Lua. And I really would want to avoid requiring luarocks and luafilesystem just for this.
• I am not sure Lua's patters are strong enough to allow easily replacing __(<anything>). In the current rakefile this is done using regular expressions.

The CONTRIBUTING file would have to be updated too, before this issue is closed.