dist.mak: allow publishing doc either from local directory or distribution archive

This patch adds a post-doc-from-dist target and restores the old behaviour of the post-doc target.
doc: adds a note on the column major / row major convention in VLFeat
vl_fisher.c: doc: add two options
vl_imwhiten.m: interface bugfix
svm.c: fix docs

Thanks to Sobhan Naderi.
small doxygen fixes
generic.h: groups together threading functions
imov.c: fix doc
kmeans: make the energy variation tolerance customizable

Previously, this test was hardcoded and differed for ANN and LLoyd.
Xcode: update project
doc: improve documentation on frames

This patch adds a short tutorial on the notion of frames. It also improves the documentation of vl_plotframe and vl_frame2oell.
vl_plotsiftdescriptor: improve doc and minor refactoring
covdet.c: fix scaling of the DoG features

This patch fixes a bug in the computation of the scale of DoG features; it also change the way the scale space is aligned in order to be equivalent to the old sift.c implementation.
setupCaltech256: fixes crash
kdtree: generate error when used with empty data
Xcode:  temporary OpenMP workaround with Xcode 5

Xcode 5 drops gcc-llvm as compiler and uses only clang. The included version of clang does not support openmp. This patch adds a build setting CC = /usr/bin/gcc to switch to a previously installed gcc version.
liop: fix memory initialization bug (closes #26)
generic: fix compatibility with older OpenMP version and improve doc

OpenMP < 3.0 did not support omp_get_thread_limit().
Xcode: upgrade schemes to 5.0
vl_covdet.m: improve doc