Merge pull request #202 from oxyplot/develop

Merge develop
iOS: cache fonts, dispose paths
#209: improve text measurements and alignment for iOS
Dispose render context in iOS PlotView
#209: improve text measurement and alignment for Android
Merge branch 'develop'
Fix output path for XamarinFormsWinPhone project
Added Gitter badge
Add gitter link
Add int overflow example
Merge pull request #211 from gitter-badger/gitter-badge

Add a Gitter chat badge to README.md
Move badge
Improve comments
#214: Make TrackerFormatStrings consistent.
Delete ITrackableSeries
Mark TrackerHitResult constructor with parameters obsolete
Add release notes
Move the Smooth property from DataPointSeries to LineSeries
Rename OxyPenLineJoin to LineJoin (breaking change)
Change LineStyle Undefined to Automatic (breaking change)
Support BrokenLineColor = Automatic in LineSeries
#214: Add missing tracker text for LineSeries and AreaSeries