Add entry and exit reactions to pystate

[TobiasPleyer]

Entry and exit reactions are very useful if there are certain things
that have to be done every time we enter or leave a state (e.g.
cleanup). Instead of making sure that every handler performs the correct
tasks this will be handled by the function transition_to.

This is less error prone, more intuitive and saves key strokes.

Changes
* The State class has 4 new methods:
- add entry_reaction
- remove entry_reaction
- add exit_reaction
- remove exit_reaction
* In addition to changing the variable current_state the function
transition_to does:
- Call the exit reactions of the old state
- Call the entry reactions of the new state