summon: a generic file launcher, needs testing on Linux and Windows

summon.py is a very simple wrapper command that abstracts the tools available in each platform for opening files from the command line. `summon bla.xyz` will open `bla.xyz` in the  default program for files of type `xyz`. If it's a html page it will open in your standard browser, if it's a directory it will open in your file browser, if it's a PDF file it will open in your pdf reader, and so on. You can pass multiple paths, separated by spaces.

Testing on Linux:
```
1.  cd $RED_SPIDER_ROOT

2.  # if you didn't have me yet:
    git remote add julian git://github.com/the-xkcd-community/the-red-spider-project.git

3.  git fetch julian
    git checkout julian/summon
    ./setup.py
    # say yes when it asks whether you want to install

4.  rsshell
    # log out of your computer and log back in if the command can't
    # be found

5.  summon ~/.profile
    # see many duplicate lines of the form "export $RED_SPIDER_ROOT=..."?
    # go ahead and delete the duplicates!
    # (this happens if you run setup.py often and is due to its somewhat
    # primitive mechanics)

6.  summon . License.txt other/spider.svg
    # the last one will not open if you don't have an SVG editor
    # the first two should appear correctly in any case

7.  exit
```

Testing on Windows:
```
1.  cd %RED_SPIDER_ROOT%

2.  # if you didn't have me yet:
    git remote add julian git://github.com/the-xkcd-community/the-red-spider-project.git

3.  git fetch julian
    git checkout julian/summon
    setup.py
    # say yes when it asks whether you want to install

4.  rsshell
    # log out of your computer and log back in if the command can't
    # be found

5.  summon License.txt
    # should open in your default text editor

6.  summon . Authors.txt other\spider.svg
    # the last one will not open if you don't have an SVG editor
    # the first two should appear correctly in any case

7.  exit
```