Parse ROLL_PTR as an integer; Modularize read_bits_at_offset

Change parsing of the ROLL_PTR 7-byte struct to parse as an integer in
order to allow parsing of roll pointers from compressed integer formats
in undo logs later on. Modularize read_bits_at_offset so it can be used
to extend other classes as necessary.
Merge branch 'master' of https://github.com/jeremycole/innodb_ruby