Add environment-specific seeds and load plugin-seeds

The commit adds a mechanism to load seeds that are specific for the
given environment. The seeds are stored in db/seeds/<environment>.rb
and should be written idempotent, to make sure, they don't
destroy/overwrite anything when being run multiple times.

The seeding now also loads the seeds for the plugins, which allows
plugins to add plugin-specific constants to the database.
Removed overwritten mail method and explicitely
removed html parts while collecting if the
Setting only permits plain text mails
Fix cuke steps "field should (not) contain" for JS-modified textareas

field.text doesn't reflect changes made by JavaScript, but field.value does.

Tested for textarea with WebRat and Selenium drivers.
Merge pull request #336 from opf/feature/rails3_rewrite_usermailer_format

Removed overwritten mail method and explicitely
removed the duplicate comment [skip ci]
Merge pull request #335 from opf/feature/rails3_enviromental_seeds

Add environment-specific seeds and load plugin-seeds
Merge pull request #338 from opf/feature/fix-cuke-field-value-step

Fix cuke steps "field should (not) contain" for JS-modified textareas
Add option to create PDFs from issue lists with a description

This adds an additional link below the issue list, called "PDF with Descriptions"
Merge pull request #344 from opf/feature/pdf_issue_list_with_description

Add option to create PDFs from issue lists with a description
Merge remote-tracking branch 'opf/feature/rails3' into feature/squashed-migrations

Conflicts:
	db/seeds.rb