Add 2 network charts to page header

-Hashrate chart is populated from get_hashrate
-Difficulty chart is populated from get_difficulty
-Added a networkhistories collection to hold the new network data
-Index sync and resync functions now check for and update network history data at the end of the sync process
-restore_backup.sh and delete_database.sh scripts now drop the new networkhistories collection
-Added a new javascript library chartjs-plugin-crosshair which adds crosshair functionality to the new chart.js line charts
-Added new settings to settings.json for controlling things like collecting network_history data, independently enabling/disabling the network charts and changing colors of various aspects of the charts
-Updated README to introduce the network charts and added a line for the chartjs-plugin-crosshair library

models/networkhistory.js

@@ -0,0 +1,10 @@
+var mongoose = require('mongoose'),
+   Schema = mongoose.Schema;
+
+var NetworkHistorySchema = new Schema({
+  blockindex: {type: Number, default: 0, index: true},
+  nethash: { type: Number, default: 0 },
+  difficulty: { type: Number, default: 0 }
+}, {id: false});
+
+module.exports = mongoose.model('NetworkHistory', NetworkHistorySchema);