Implement belongs_to and has_and_belongs_to_many

[saki7]

Quoting from #71:

one_to_one isn't explicitly supported, you can achieve it with having a has_one attribute within each entity. But its not exactly what one_to_one stands for.

I'm afraid that has_one + has_one is logically incorrect.
The possible scenarios are only limited to following:

• has_one + belongs_to = "Game has one Game Community, Game Community belongs to one Game"
• has_many + belongs_to = "Post has many Attachments, Attachment belongs to one Post"
• has_many + has_many = "Post has many Categories, Category has many Posts" (need a joining table)

For OOS 0.5.1.

[zussel]

Btw: Good summary about the relationships!

By now only has_many with no counterpart ist implemented 😞

But w'll change that!

[zussel]

I came to the conclusion that at least belongs_to is a major feature that shouldn't be missing. Therefor I attached it to milestone 0.5.0.