Consider renaming actual grunt task name and config option to 'casper'?

[jsoverson]

Might be me rushing through docs, but I expected the task name and config to be 'casper' instead of 'casperjs' due to the name of the npm module.

'casperjs' totally makes sense though, so I might be way off base.

[iamchrismiller]

Changed Task name from casperjs -> casper for task clarity.