covdet: substantially improve the documentation

The documentation of this module was long overdue. This patch adds a significant amount of documentation, although it is not complete yet.
vl_phow.m: improve documentation
webdoc.py: improve index generation

Previously, only <a> tags would get the class=‘active’ attribute in generated indexes; Now the same attribute is applied to the containing <li> element too.
fisher.c: fix typos in the documentation
slic.c: minor doc fix
vl_sift.c: minor doc fix
mser.c: fix a Doxygen warning
webdoc.py: improve navigation
website: new layout
0.9.17 -> 0.9.18
hikmeans: fix compiler warnings
covdet.c: revert bug accidentally introduced in previous patches
Xcode: fix an incorrect flag for a file
misc: cosmetic fixes
compiling.html: cosmetic fix
fisher.c: complete the implementation of the FAST option
some release notes
lbp.c: windows fix
Makefile.mak: sync
test_sqrti.c: windows fix