Don't link to harvest reports on issue's page unless current user is allowed to view harvest reports
Added permission "See harvest time"

User can see Harvest time attribute on issue's page only if he's allowed
to.
Using Harvest gem instead of Harvestr + some refactoring

Previously used Harvestr gem relies on Mash gem which causes the following bug:
http://github.com/mully/redmine_harvest/issues#issue/2
Merge remote-tracking branch 'szajbus/master'

Conflicts:
	app/models/harvest_time.rb
	init.rb
	test/test_helper.rb