Dot not assume Table._rows will always be in memory

[turicas]

Currently some operations assume all rows are in memory (such as order_by). We may move all the code to something lazy.
For order_by specifically we could sort in-disk instead of in-memory, like csvsort does.

• Implement LazyTable class
• Add lazy parameter to rows.plugins.utils.create_table
• Change all plugins to force (or not) lazyness (example: in HTML
  lazy=False, in CSV lazy=True by default)
• Change default sample size to 1000 (or any other non-arbitrary number)
• Create documentation about it

[turicas]

Working on branch feature/lazyness.