add a convenience method for creating nested groups in Property

It is cumbersome to create groups in code using yarp::os::Property.
Added yarp::os::Property::addGroup to alleviate the pain a little.
A real fix requires a pending clean-up of the implementation.
Thanks @stephane-lallee for bringing this up.
changed order of tutorials for better separation of what is relevant for developers and what is relevant to understand YARP implementation
Fixed some documentation issues
Fixed some warnings
[gaze I/F]: just doc updated
Merge branch 'master' of github.com:francesco-romano/yarp into francesco-romano-master
Merge branch 'francesco-romano-master'
reporting error code on thread failure
using sprintf istead of itoa
added to yarp write completition a single history file for each different port
Used resourceFinder to get the history folder path, disable history logging if yarp write is used in a pipe
yarp write history: ACE is used instead of normal posix functions
Portability added to yarp write history
Merge pull request #180 from MirkoFerrati/master

added to yarp write completition a single history file for each differen...
patch up for YARP-without-ACE build
make readline code conditional on readline library
remove some cruft from tcpros_carrier
fix typo in carrier argument
give bottle version of yarpidl_rosmsg managed structures
add missing header files to list, fixes #23, thanks @iron76