Make BUILDTYPE=Release the default if no make parameters are specified

This probably simplifies the default case for most users of the
library, and reduces the risk of people accidentally trying to use
an unoptimized build.
Update the description of the directories in the readme

The processing directory has been moved under codec, and the
main recommended build process doesn't produce anything under bin.

Additionally mention the res directory which is where the test
sequences have been moved.
Include the executable suffix within make

This makes sure that e.g. "make clean" actually removes
the built exe files, and avoids relinking the exe files each
time make is run when cross compiling from linux to windows.

(Make on windows seems to have an exception that knows to skip
rebuilding the 'h264enc' target if 'h264enc' doesn't exist but
'h264enc.exe' does, but this exception doesn't work on normal
unix make.)
Merge pull request #197 from mstorsjo/update-readme-dirs

Update the description of the directories in the readme
Merge pull request #199 from mstorsjo/make-exeext

Include the executable suffix within make
Merge pull request #196 from mstorsjo/build-release-default

Make BUILDTYPE=Release the default if no make parameters are specified