Improved Composer autoload configuration.

This PR utilizes Compser's [files](https://getcomposer.org/doc/04-schema.md#files) autoloading feature which is designed for functions such as those provided by this library. This means Composer users will no longer have to manually require the Hamcrest functions.

In addition, I changed the autoload configuration to only generate a class map for this library's dev resources when in development, as the current setup is adding cruft to end users' autoloaders.

I also updated the readme to contain information about the Composer package.