cache body height to prevent unnecessary calculations

Scrolling performance in UITableView is a major issue because
tables can get very large and because smooth scrolling is a
desirable effect. This is one of several optimizations
that help enable smooth scrolling experience.
Refactor

Turn requiredBodySize into a lazy property
Organize constant section
Bugfix

Work around an annoying bug (in OHAttributedLabel?) where y-value of bodyHtml bounds
would go in the negative direction if frame origin y-value exceeded
44 pts (due to 44-pt toolbar being present?)
Merge pull request #26 from Livefyre/dev

Bugfix + table scroll performance improvement