Initial commit.
Add test script for each problem separately

The advantage of having a separate script for each problem (instead of
all the tests in one core.clj script) is that can you see the progress
while running "lein test".

Another advantage is that you can test just a selection of problems by
running lein test and give a list of test namespaces. For example:
"lein test project-euler-solutions.test.problem-0001".
Add test script for and doc strings to common
Change solution of problem 10 using multiple threads

The solution of problem 10 can be subdivided in several independent
tasks.  This solution uses a Java Executor with a fixed thread pool to
run the tasks. The pool size equals the number of available processors
(cores). The number of tasks were determined just by trying.
Upgrade to Clojure 1.3.0
Add solution for problem 15
Make binomial-coefficients of problem 15 lazy
Add solution for problem 16
Add solution for problem 17
Make functions of problem 17 private
Add solution for problem 18
Add solution for problem 19
Add solution for problem 20
Add solution for problem 21
Add solution for problem 22
Upgrade to Clojure 1.4.0
Add solution for problem 23
Change solution of problem 23 using multiple threads
Update .gitignore
Change divisor? to speed up some of the solutions

In particular the solution for problem 23 will be a little bit faster.
The divisor? function now expects just one divisor to check, not a
variable list of arguments.

The 'old' divisor? function is now called some-divisor and will return
the first divisor (if any, else nil) instead of true or false.