Simplify folder hierarchy

I reduced the number of sublevels and clarified that organizations
are typically represented as websites where documents are published.
Add source for IPCC AR1 WG1

Source:
http://www.ipcc.ch/publications_and_data/publications_ipcc_first_assessment_1990_wg1.shtml
Add Appendixes 2, 3, 4, 5 of AR1 WG1
Add source for AR1 WG2
Add source for AR1 WG3
Add sources for AR2 WG1, WG2, WG3
Clarify top level folders

They correspond to websites where source documents are published,
which may differ from the website of the organization which
issued the documents initially.
Add ipcc in the name of folder for IPCC assessment reports
Add sources for AR3 WG1, WG2, WG3
Add sources for AR4 WG1, WG2, WG3
Add sources for AR5 WG1, WG2, WG3