Moved classes to src directory

Apparently, using a “src” directory is a better way to organize a
package.
Moved files to PSR-0 compliant directories

Now that they’re in the src directory, the files should be in a
standard PSR-0 compliant directory structure (i.e.,
<VendorNamespaceName>(\<SubNamespaceNames>)*\<ClassName>)
Updated namespace and package attribution

Before the files were in the “Jsc” namespace with a package attribution
of “Jsc/php-helpers”. I fixed both to “Jsc/PhpHelpers”.
Updated namespace and package attribution (again)

It would probably be easiest if my vendor namespace matched my GitHub
user name. Final answer.