gradle: update buildtools to 20.0.0

Change-Id: I1d166f9c381b60b5df5a726c73a26f372e244196
replace prebuilt libs with gradle dependencies

MPChartExample depends on MPChartLib, so the support v4 is already
included and we do not need to specify it a second time

replace the nineoldandroid library with a gradle dependency

Change-Id: I9e4b3df7b243153057a382c410668d1633eb42ec
some refactorings and fixes

Change-Id: I4df752641356375df1e607655566ce7e511477bf
allow to override paint colors, allow to disable touch on pie charts

Change-Id: I6a1176ebe65e264fab70791ddb126a496a3ace4a