Fix that bug where only one line was loaded

Let me tell the story backwards:
For some reason, only one line would be fetched in `fetchMoreLines`, but
neither `numLines` nor `buffer.requestedLines` were `undefined`. The
paramater `numLines` must have had some strange value, though.
`connection.fetchMoreLines` is invoked through the main controllers
`$scope.fetchMoreLines`, which sets the parameter to `$scope.lines` if it was
undefined before. That value is computed in `$scope.calculateNumLines`,
which takes the height of the lines area and divides it by the height of the first
bufferline. This computation is retriggered on every `resize` event.
The first bufferline is the 'fetch more lines' link at the top of the window. If the
currently active buffer doesn't have more lines, it is hidden with `ngHide`,
causing its `clientHeight` property to be `0`, and the number of lines to be
fetched `$scope.lines = Infinity` (due to a division by zero).
In `connection.fetchMoreLines`, the following request is then made:
`"buffer:0x" + buffer.id + "/own_lines/last_line(-" + numLines + ")/data"`
to which WeeChat responds with one line. Voilà, there's your mess.
Merge pull request #282 from glowing-bear/fixonelinebug

Fix that bug where only one line was loaded