Implemented help command.

If optional command is passed - print this command's help. If not -
print common help.
Help module refactoring
Implemented versioning support.

Version is extracted through 'git describe' (using latest tag).
Add 'install' target for installing embox binary.
Fix version extraction from 'git describe' command.
Fix version extraction in case of multidigital numbers.
Added 'cat' module.