Moved exercises into subfolders of the corresponding topics

W. Trevor King: I dropped everything from the original 39e0701 except
for the python/testing/exercises/test.markdown rename.

Conflicts:
python/data_structures/exercises/setdict.markdown
python/data_structures/exercises/setdict_dependencies.txt
python/intro/exercises/python.markdown
python/obj_orientation/exercises/oop.markdown
python/obj_orientation/exercises/oop_sushi.txt
python/regexp/exercises/regexp.markdown
python/sw_engineering/argparse_exercise.md
shell/exercises/1000gp.vcf
shell/exercises/shell.markdown
spreadsheets/exercises/spreadsheets.markdown
spreadsheets/exercises/spreadsheets_ex1.xls
spreadsheets/exercises/spreadsheets_ex2.xls
spreadsheets/exercises/spreadsheets_ex3.xls
sql/exercises/aggregation.markdown
sql/exercises/experiments.sqlite
sql/exercises/experiments_null.sqlite
sql/exercises/experiments_subqueries.sqlite
sql/exercises/filter.markdown
sql/exercises/intro.markdown
sql/exercises/join.markdown
sql/exercises/nobel_prizes.sqlite
sql/exercises/null.markdown
sql/exercises/select.markdown
sql/exercises/sort.markdown
sql/exercises/subqueries.markdown
version-control/svn/exercises/exercise_4.cgi
version-control/svn/exercises/planets.txt
version-control/svn/exercises/svn.markdown
version-control/svn/exercises/temperature.txt

Adding exercises from the Software Carpentry 4.0 website

W. Trevor King: I dropped everything from the original da170d3 except
for the exercises/test.markdown addition.  From Jon's initial pull
request [1]:

  Here are a bunch of exercises, markdown-ized, that we used to have
  as part of the website for various topics.

  I wasn't sure where to put them exactly, so I've created a new
  top-level folder for them, and placed them there, flatly. I've named
  things so that the assets for a page of exercises have the same file
  name prefix (e.g. vc.markdown, and vc_planets.txt).

[1]: https://github.com/swcarpentry/boot-camps/pull/34