Update tutorials.rst

Change some wording
Update table_of_content_introduction.rst
ability to compile for iOS with Neon
yet another attempt to refactor features2d; the first commit, features2d does not even compile
Implementation detector and selector for IPP and OpenCL;

IPP can be switched on and off on runtime;

Optional implementation collector was added (switched off by default in CMake). Gathers data of implementation used in functions and report this info through performance TS;

TS modifications for implementations control;
Merge pull request #3335 from ilya-lavrenov:neon_ios
Merge pull request #3333 from micha137:patch-1
Merge pull request #3334 from micha137:patch-2
OpenCV with the refactored features2d compiles! contrib is broken for now; the tests are not tried yet
Applied proposed fix
fixes
Merge pull request #3270 from boaz001:patch-1
Merge pull request #3338 from ilya-lavrenov:ios_fixes
now all the samples and opencv_contrib compile!
fixed several test failures; currently 9 out of 73 tests fail
Merge pull request #3332 from lupustr3:pvlasov/implementation_detector
all the tests now pass except for MSER
ok; all the tests now pass
fixed opencv_java build
trying to fix strange compiler bug