Changes in DBIx::Tree::NestedSet 0.11

Removed the DBI::disconnect() and "unlock tables" statement in DESTROY(). Oops.
Added some missing DBI::finish()es.
Abstracted out the RDBMS-specific stuff into separate driver classes. See "DBIx::Tree::NestedSet::MySQL" for an example. Should make "porting" to different RDBMSs very easy.
Created "DBIx::Tree::NestedSet::MySQL" and "DBIx::Tree::NestedSet::SQLite" driver classes. If you wanted to create a driver class for another RDBMS, you should probably look at source of the MySQL class first.
Created "create_default_table" and "get_default_create_table_statement" methods that call to the RDBMS driver classes.
