v1.5.2 released

v1.5.2 of timetools is released.

It introduces:
- updates in man pages ;
- definition of barplot methods for SubtimeDataFrame, TimeIntervalDataFrame and TimeInstantDataFrame ;
- enhancement in split methods ;
- more detailed documentation of tapply and split sources ;
- removing of the use of roxygen for documentation (that will be completed later).

See README for details.