Add AGAST feature detector

 This PR implements AGAST detector [1].

Reasons:

 * AGAST is faster than FAST, and is an good alternative to have among FAST and Harris.
 * AGAST has much better scoring routines, also have more testing/scoring patterns.
 * Some other detector e.g. BRISK depends on it see [2] and [3] bug reports.
 * Original code ported here is BSD license (compatible), and can be found at [4].
 * This PR also updates test, perftest and doc, i tried to follow actual FAST implementation 1:1
 * This PR has all proper citation in respect with the authors.
 * This PR follows opencv of my very best try, i hope it gets accepted at all.

[1] http://www6.in.tum.de/Main/ResearchAgast
[2] http://code.opencv.org/issues/3976
[3] http://code.opencv.org/issues/2491
[4] http://sourceforge.net/projects/agastpp/