utf8mb4 and database creation

Recreated pull-request from new clone of Elgg/Elgg.

The utf8mb4 encoding introduced in MySQL 5.5.3 allows for the 4-byte emoticons characters which can be rather nice in a social website like Elgg. This commit allows selection of either utf8 or utf8mb4 at installation time.

Also, if users prefer to have the database created by Elgg as part of the installation, this commit allows for that - superuser name and password for the database server should be provided and will only be used for connecting and creating the database. The information is not stored in the settings file. The given database user will also be created and granted permissions to the new Elgg database.