title: gpxpy doesn't parse time attribute of track points
body: <p>i'm trying to parse gpx file made by "Mission Planner". For some reason the softwere generates the gpx file of one line, which look like that:</p>
  
  <pre><code>&lt;gpx creator="Mission Planner 1.3.48 build 1.1.6330.31130 ArduPlane V3.7.1 (22b5c415)" xmlns="http://www.topografix.com/GPX/1/1"&gt;&lt;trk&gt;&lt;trkseg&gt;&lt;trkpt lat="31.7562743" lon="35.1812861"&gt;&lt;ele&gt;719.5&lt;/ele&gt;&lt;time&gt;2017-06-13T20:08:28+03:00&lt;/time&gt;&lt;course&gt;113.6&lt;/course&gt;&lt;roll&gt;-165.05&lt;/roll&gt;&lt;pitch&gt;1.74&lt;/pitch&gt;&lt;mode /&gt;&lt;/trkpt&gt;&lt;trkpt lat="31.7562703" lon="35.1812854"&gt;&lt;ele&gt;723.3&lt;/ele&gt;&lt;time&gt;2017-06-13T20:08:29+03:00&lt;/time&gt;&lt;course&gt;94.72&lt;/course&gt;&lt;roll&gt;-168.73&lt;/roll&gt;&lt;pitch&gt;8.55&lt;/pitch&gt;&lt;mode /&gt;&lt;/trkpt&gt;&lt;trkpt lat="31.7562648" lon="35.1812912"&gt;&lt;ele&gt;725.2&lt;/ele&gt;&lt;time&gt;2017-06-13T20:08:30+03:00&lt;/time&gt;&lt;course&gt;86.72&lt;/course&gt;&lt;roll&gt;-172.74&lt;/roll&gt;&lt;pitch&gt;4.67&lt;/pitch&gt;&lt;mode /&gt;&lt;/trkpt&gt; (...)
  </code></pre>
  
  <p>When i try to parse the gpx file with gpxpy (gpxpy-1.1.2), the track points have an empty time attributes, even though they do have have a  field in the gpx file:</p>
  
  <pre><code>In [76]: a = gpx.tracks[0]
           b = a.segments[0]
           c = b.points[1]
           d = [c.longitude, c.latitude, c.elevation, c.time]
           d
  
  Out[76]: [35.1812854, 31.7562703, 723.3, None]
  </code></pre>
  
  <p>obviously, that kills all the speed / duration calculations.</p>
  
  <p>ideas? suggestions?
  + if anyone knows a parser / script that can change the gpx file to be readable, that would be nice. i'v tried to write one of my own, but the \n i insert somehow fuck up the gpx file, the gpxpy cannot parse it at all.</p>
  
  <p>thx ahead,
  Forest.</p>