If aspell is found on the system use it for spellchecking.
Otherwise use GoogleSpell.

Bug#812030

Also renamed the error() function in htdocs/js/tinymce/plugins/spellchecker/includes/general.php
to mceError() because it conflicts with the mahara defined error() function.

Change-Id: I71f17cb29c7dfcd5ea9731c2c1b8e2a64fcc46ef
Signed-off-by: Christopher Tombleson <christopher@catalyst.net.nz>