whitespace fixes
patches nested_set to add tree rebuilding methods that are not using
save

The original nested_set implementation uses save on each node (issue,
project). With the callbacks defined on issues, this can lead to stale
object errors when an issues has been loaded by the rebuilding method
but changed by a callback.

The rebuild_silently! method can also take root nodes as their
parameter. With such provided the method will only fix their trees.

This is build upon by the selectively_rebuild_silently! method which
first looks for invalid nodes, determines their root node and then calls
rebuild_silently! with those root nodes. This should speed up rebuilding
on large trees.

Additionally, methods where added as a byproduct to retreive nodes
who's tree attributes are invalid.
overrides awesome_nested_set methods for issues to allow for fixing the
root_id

The root_id provides the scope for a nested_set tree. The implicit
constraint of the Issue implementation is that there is but one tree per
scope. Or in other words, the root_id is part of the nested_set
attributes for an issue.

This is not the case for Project, which is also using nested_set but
does not set a scope. Project's are ordered that way.

The root_id should always point to the root of an issue tree.
awesome_nested_set only looks for nodes without a parent in the provided
scope to determine root nodes. For example, if the root_id of an issue
should point to the node itself, this issue is considered to be a root
node. This is regardless of the value of the parent_id. For such a node,
nested_set methods will provide wrong issues when queried for root,
ancestors and similar methods.

Fixing a missconfigured root_id attribute is achived by traversing up
the parent chain (parent_id) till an issue without a parent is found.
The parent_id attribute is considered to be correct.

The root_id is not really necessary for awesome_nested_set to work. But
employing the field should be beneficial performancewise. Updating an
issue tree requires less nodes' left/right attribute to be altered when
the forrest is scoped.
Merge pull request #5 from opf/feature/silently_rebuild_nested_set

Feature/silently rebuild nested set