Add utility function for temporary modifying classpath
Add logging utility and test utility functions
Improve logging testutils, and move testutils into src

This commit has two different parts:

1. It adds some improved macros to the logging testutils; with the
   new `with-test-logging`, test authors do not have to manage
   the state of the log entries atom, and can use the `logged?`
   assertion expression with `clojure.test/is`.

2. It moves all of the testutils code out of the `test` directory
   and into the `src` directory.  This was done because leinengen
   (correctly) does not include the `test` source code in the jar
   file that it packages when, e.g., doing a `lein deploy`; however,
   the testutils code is extremely useful for test authors who
   probably already have a dependency on clj-utils.  In the future
   we may want to look into building a separate "test jar"; maven
   has some support for this, but I didn't immediately see the
   equivalent functionality in leinengen.
Merge pull request #1 from cprice404/feature/master/add-classpath-and-test-utils

Feature/master/add classpath and test utils