- Eager loading now functions at any arbitrary depth along self-referential

-  Eager loading now functions at any arbitrary depth along self-referential
and cyclical structures.  When loading cyclical structures, specify "join_depth"
on relation() indicating how many times you'd like the table to join
to itself; each level gets a distinct table alias.
- adds a stack to the Mapper->eagerloader->Mapper process which is the single
point of tracking the various AliasedClause objects both at query compile
time as well as result fetching time.  self-referential relationships narrow
down the "aliasing" of tables more sharply so as to produce the correct
eager joins in those cases without stepping on more generalized cases.
the mechanism of detecting "too deep" of an eager load now works based on
locating a true cycle, but only if join_depth is not specified; otherwise
join_depth is used.
[ticket:659]