Optional serde support (#37)

This PR makes serde an optional feature. 

I also added serialization derive traits to `BT` struct so you can now
serialize the whole behavior tree.

refactor adding WhileAll behavior (#36)

**In this MR**

* Rename behavior  `RepeatSequence` to `WhileAll`
* Improve readme slightly for While and WhileAll behavior
* Bump rust version
* Bump lib version

Separate graphing data from main BT struct (#32)

All data needed to draw graphs is now created on the fly every time when
`get_graphviz()` gets called. This should make behavior trees consume
less memory and construct faster.

Issue: #29

Generic blackboard implementation (#26)

Generalize blackboard. 

From user's perspective, in addition to being able to use generic
blackboard type, the user must now use tick directly with `bt.tick(...)`
instead of `bt.state.tick(...)` This change is required so that we can
pass a mutable reference of the blackboard to the user's tick callback
under the hood.

fix ci badge

fix failing doctest

Merge pull request #13 from Sollimann/release-0.3.9

Release 0.3.9

typo in docs

newline

push images