[common.logstreams] update memory handling

This commit should get rid of some warnings and small inconsistencies.
The LogStream class now derives from StorageProvider which holds the
buffer before the buffer is given to the ostream base class. FileLogStream
and EmptyLogStream now merely create the corresponding stream buffer
and forward it to LogStream. Thus overriding rdbuf does not seem necessary
any more.
[test.common_logger] added "test" for file logging.

This does not yet test whether the file has actually been created or
if it contains the correct stuff.
[common.logstreams] added first draft of TimedPrefixedLogStream
[test.common_timedlogging] added first draft
[common.color] put some stuff into lib
[common.color] removed bold_white()
[common.color] added color_map()
[common.logstreams.timedprefixedlogstream] do not alter given prefix
[common.logstreams] added documentation
[common.logging] added TimedLogManager and TimedLogger()
[test.common_timedlogging] update