Reorganized code for easier viewing/less folders.

Reorganized code to use one directory instead of needing two directories
and an xcodeproj folder for every problem.

Currently runs all the problems, lines must be commented out in main in
order to run specific problems by themselves. This should be changed to
accept an optional argument to run individual tests.
Rename files to proper Euler Problems.

Euler4 was actually Problem 7, Euler7 was 8, Euler1 was 3.
Correct return types of functions in problems.h

Return types have changed to void, reflect that change in the header.
Implement function array and user input.

Euler functions can be added to the array and specific problems can be
run based on user input or by changing the runProblem variable.
Disable user input thing for now.

Easy enough to reactivate but it might be a bit overengineered for this
point.
Use environment variable for txt location

This allows it to be a bit more portable, though relative paths
would be nicer if thats an option.
Merge pull request #1 from zolrath/master

Reorganized code for easier viewing/less folders.