Prevent gliders on ticks.
Implement and add quad tree

Quadtrees can be used to speed up Curve.hasPoint(). See http://jsxgraph.uni-bayreuth.de/~michael/jsxgui/Examples/internal/qdt.html for an example.
Introduce broad search step in Curve.hasPoint for plots and functiongraphs

This broad search is implemented using quad trees. Instead of running through all of the curve's datapoints we first collect all points in a quad tree and use this tree to search for a very small number of points (atm 10) which are then used to check if the given point is near the curve. This reduces the number of nearance checks drastically, the Curve.hasPoint() runtime no longer depends on the position of the given point.

The quad tree is only used if the curve attribute useQDT is set to true (default for now: false) and a tree has been calculated. The tree is updated in updateCurve() but only if the board quality is set to BOARD_QUALITY_HIGH.
Remove private helper classes AABB and XY from math/qdt module
Allow calling the UpListener directly without an event object
Update fill attributes for lines

Apparently, some pdf renderers (e.g. firefox) treat undefined fill color/opacity as grey which leads to undesired frames for lines in pdf exports. This closes #77.
Bug fix: very short arrows (< 10px)
More tests with arrow heads
First commit  with new computer
New element: stepfunction
Doc for stepfunction
Another bug fix in doc of stepfunction
More docs for stepfunction
Bug fix: return value of Geometry.projectCoordsToSegment()
First step to implement HTML slider.
Work on HTMLslider
Work on HTMLslider
Make sure the last set time is stored in any case
Vertical alignment of HTMLslider range element
Merge branch 'master' of github.com:jsxgraph/jsxgraph