Fix a docs typo
Allow using low-level sockets as Rserve handles
Create a WebWork macro

The macro allows use of this module from the WebWork automated
homework grading system: http://webwork.maa.org
Switch from Moo to Moose (fixes #22)
Add an option to create a low-level socket connection to Rserve

This is necessary for use in Webwork because IO::Socket::INET
interferes with Webwork's templating when rendering a problem.
Better management of Rserve connection in the WebWork macro

- rserve_start always creates a new session

- rserve_finish undefs the variable holding the Rserve session so a
  subsequent rserve_eval knows to create a new one if the user
  ommitted a call to rserve_start

- rserve_query explicitly closes the connection when it's done for an
  immediate cleanup
Install 'extras' as the distro's 'share' directory

This way the Rserve macro can be installed by copying it from
`perl -MFile::ShareDir -e 'print File::ShareDir::dist_file("Statistics-R-IO", "WebWork/RserveClient.pl"`
Use custom Moose type constraints for attributes (fixes #23)