Update Wms_Install.php

Fixed to run with table_prefix. I've tested it on Ushahidi 2.7.4, more test coming
Update wms_layer.php

Added support to table_prefix. Tested in Ushahidi 2.7.4
Update wms_settings.php

Added teble_prefix support. tested on Ushahidi 2.7.4
Merge pull request #5 from Loz83/patch-1

Table_prefix & Ushahidi 2.7.4