Add 'decorator'[1] package to Buildroot. Needed by 'traitlets'
package, which is added in the next patch.
[1] https://pypi.python.org/pypi/decorator
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
[Thomas: download from PyPi, add hash file, add entry in DEVELOPERS file.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
