Formatters update, bugfixes

• PieChart and HorizontalBarChart now support MarkerView
• Dashed highlight-lines now possible
• Introducing FillFormatter for individual LineDataSets
• Introducing XAxisValueFormatter for custom XAxis label formatting
• Introducing YAxisValueFormatter for custom YAxis label formatting
• Formatter package name changed (from utils to formatter)
• Documentation wiki improvements
• General improvements
• General bugfixes