Method iter_subtrees_topdown added

Allow to passing "rankdir" to lark/tree.py:pydot__tree_to_png

lark/tree.py

shaping. For example, passing "TB" instead of "LR" makes the tree
to be draw vertically instead of horizontally.

https://stackoverflow.com/questions/29003465/pydot-graphviz-how-to-order-horizontally-nodes-in-a-cluster-while-the-rest-of-t

docs: clarify end_column value

I interpreted "the column where the token ends" to mean "a pointer
to the last character in the token", which would be the same as
the start column for a single-character token.

However, that's not what lark actually reports. Reword to clarify
this.

https://github.com/lark-parser/lark/issues/240

Added missing binary files to .gitignore

# Conflicts:
#	.gitignore

Typo in documentation

Fix a small typo

Fix Lark class docstring

It should be easy to allow the user to choose between them, but perhaps unnecessary, as the one and only benefit of the traditional parser is a tiny performance advantage.

Fix typo in how_to_use.md sample code: gramamr -> grammar

Related to issue #216