restructure website

1. no hyperlinks to harvard.edu
2. new directory structure:

   signaling
   |-- explore
   |   |-- cell_line
   |   |   |-- 184B5.html
   |   |   |-- ...
   |   |   |-- ZR-75-30.html
   |   |   `-- img
   |   |-- ligand
   |   |   |-- BTC.html
   |   |   |-- ...
   |   |   |-- VEGF165.html
   |   |   `-- img
   |   `-- signaling_response_matrix
   |       |-- img
   |       `-- index.html
   `-- home.html
fix bug in sibling_browser.html
replace dead buttons with nav buttons
push .etc under explore
rename home.html to start.html
add legends to cell_line & ligand pages

closes issue #10