Add extra padding to the bottom of an editor's vertical scrollbar

This gives us some space when we scroll to the bottom of a file.
Similar to how MacVim works when using the mouse.

Closes #464

spec/app/editor-spec.coffee

@@ -317,7 +317,7 @@ describe "Editor", ->
         expect(editor.charWidth).toBeGreaterThan charWidthBefore
         expect(editor.getCursorView().position()).toEqual { top: 5 * editor.lineHeight, left: 6 * editor.charWidth }
         expect(editor.renderedLines.outerHeight()).toBe buffer.getLineCount() * editor.lineHeight
-        expect(editor.verticalScrollbarContent.height()).toBe buffer.getLineCount() * editor.lineHeight
+        expect(editor.verticalScrollbarContent.height()).toBe (buffer.getLineCount() + editor.bottomPaddingInLines) * editor.lineHeight
 
         newEditor = new Editor(editor.activeEditSession.copy())
         editor.remove()
@@ -1297,6 +1297,7 @@ describe "Editor", ->
 
     describe "when lines are removed", ->
       beforeEach ->
+        editor.bottomPaddingInLines = 0
         editor.attachToDom(heightInLines: 5)
 
       it "sets the rendered screen line's width to either the max line length or the scollView's width (whichever is greater)", ->
@@ -1391,6 +1392,7 @@ describe "Editor", ->
     describe "when autoscrolling at the end of the document", ->
       it "renders lines properly", ->
         editor.edit(project.buildEditSession('two-hundred.txt'))
+        editor.bottomPaddingInLines = 0
         editor.attachToDom(heightInLines: 5.5)
 
         expect(editor.renderedLines.find('.line').length).toBe 8

src/app/editor.coffee

@@ -64,6 +64,7 @@ class Editor extends View
   newCursors: null
   newSelections: null
   redrawOnReattach: false
+  bottomPaddingInLines: 10
 
   # Public: The constructor for setting up an `Editor` instance.
   #
@@ -1280,7 +1281,10 @@ class Editor extends View
       @overlayer.height(height)
       @layerHeight = height
 
-      @verticalScrollbarContent.height(height)
+      bottomPaddingInLines = if @mini then 0 else @bottomPaddingInLines
+      heightWithPadding = height + (@lineHeight * bottomPaddingInLines)
+      @verticalScrollbarContent.height(heightWithPadding)
+
       @scrollBottom(height) if @scrollBottom() > height
 
     minWidth = @charWidth * @maxScreenLineLength() + 20
@@ -1516,7 +1520,8 @@ class Editor extends View
   #
   # Returns a {Number}.
   getLastVisibleScreenRow: ->
-    Math.max(0, Math.ceil((@scrollTop() + @scrollView.height()) / @lineHeight) - 1)
+    calculatedRow = Math.ceil((@scrollTop() + @scrollView.height()) / @lineHeight) - 1
+    Math.max(0, Math.min(@getScreenLineCount() - 1, calculatedRow))
 
   # Public: Given a row number, identifies if it is currently visible.
   #