[lua binding] remove ulatency.get_uid_stats()

Has no usage since we track real sessions.
[core] Fix opening U_SESSION lua lib
Fix inline functions
[docs] Rename README to README.md
[docs] Convert README.md content to markdown

And fix some typos.
[build] Configure flags for build types and use link time optimization

For each build type flags are initialized following:

Debug: 		-g3 -pg
Release: 		-O3 -fomit-frame-pointer -flto -DNDEBUG
RelWithDebInfo: -g -O2 -finline-functions -flto
MinSizeRel:		-Os -flto -DNDEBUG

Always add "-Wall -Wformat -Werror=format-security" in front of
CMAKE_C_FLAGS.

Make linker use same flags used by compiler; link time optimization
needs this.
[build] Set default BUILD_TYPE to "Release"
Add ulatencyd --version option, which prints (some) build options
[mappings] Consider process w/ user.idle being active only if focused

This allows a process which creates X window to be labeled with
user.idle. If the window is focused, the process is scheduled as
active; if not, it is scheduled as user.idle.

Former behaviour was to consider as active all processes that are
in the focus stack, regardless their position and without taking
user.idle label into account.

This commit changes this behaviour:

A process which is tagged with user.idle flag is moved to usr_$$/idle
cpu cgroup unless it is focused, i.e. unless it's position in the
focus stack is 1, in which case it is moved to usr_$$/active;
similar for other subsystems.
Updates for development release
Merge branch 'master' into devel