moves #duration and #reschedule_after from subclasses to work_package

This changes the default behavior as it was for issues so that the duration calculation includes start and due dates. As an example, a work package from today to today has a duration of 1. 1 is also returned if start or due date is not set.

Reschedule after is moved unchanged, even if there might be some issues and inconsistencies. E.g. it depends on the "update_parent_attributes" method to propagate the due/start. This leads to an inconsistency where a reload is required if the wp is not a leaf while no reload is required when it is.
updates changelog
Merge branch 'feature/rails3' into feature/harmonize_duration

Conflicts:
	doc/CHANGELOG.md
Corrects typos
Adds wiki version link route spec
Adds cuke for wiki links in activity
Checks only the journal for version information
Creates and old wiki content version from journal
Adds changelog entry

[skip ci]
Merge pull request #366 from opf/feature/harmonize_duration
Fixes 20130814130142_remove_documents.rb

The down migration used i18n that was not available as well as using
table_exists w/o the question mark.
Merge pull request #363 from opf/bug/rails3/wiki_activity_link_broken

[BUG] Wiki activity link broken
Merge remote-tracking branch 'opf/feature/rails3' into feature/fixed-20130814130142

Conflicts:
	doc/CHANGELOG.md
Merge pull request #368 from opf/feature/fixed-20130814130142

Fixes 20130814130142_remove_documents.rb
Merge branch 'feature/rails3' into feature/rails3_move_pe_controller_to_api_v2

Conflicts:
	doc/CHANGELOG.md

[skip ci]