Update README to remove sudo instruction 

Jeremy, I am a beginner to Ruby and have been doing a few projects with Sequel and it has been great.  I noticed in the README that the instruction for install was to use `sudo`.  It seems like this is out of date.  Similar to https://github.com/sstephenson/rbenv/issues/532, I think removing `sudo` would be a good update.
Remove the install/uninstall rake tasks

I could change these not to default to sudo, but I never use them
and see no point in having them.
Support offsets without limits on MySQL, SQLite, H2, SQLAnywhere, and CUBRID

We didn't have any tests before, so I didn't even know that
these databases don't have native support.  On SQLite and H2
you can use a limit of -1 specify no limit, but MySQL, CUBRID and
SQLAnywhere don't support the concept at all, so you have to use
a large numeric limit, and the maximum limits the databases support
are all different (MySQL 2**64-1, CUBRID 2**32-1,
SQLAnywhere 2**31-1).
Fix RDoc for null_dataset extension