Prepare for integration

Fixed spelling

Left F64 in comments, as it's already in tantivy master branch, but hasn't been
published yet

Added a todo list on our way to integrating tantivy-cli as a subcrate in core tantivy

Added score to the `serve` output.

Create target directory in `tantivy new` if it doesn't exist

Added license

Closes #34

update tantivy to 0.6.1

* remove tantivy::TimerTree code
* change Index::open() to Index::open_in_dir()
* change Index::create() to Index::create_in_dir()

* change from create() to create_in_dir()

* fix issue #12 "Field name must match the pattern"

Prevent panics when bad state is found - e.g. no meta.json or no indexing dir

Using tantivy 0.4.4