Qldbshell

:tada: Enhancements
* Added `clear` to clear the screen.
* Remove pyqldb as dependency and add driver submodule.

:bug: Fixes

* Fixed empty line repeating the last command used in the shell. Now, an empty line does not repeat the last command.

:tada: Enhancements

* Added `quit` and `Ctrl + D` as ways to exit the shell along with existing `exit` command.
* Added interactive and non-interactive transactions to the shell. More details can be found in the [release notes](http://github.com/awslabs/amazon-qldb-shell/releases/tag/v1.1.0).
* Changed shell depedency from Cmd to prompt-toolkit to pasting multiple lines and entering multiple lines. More details can be found in the [release notes](http://github.com/awslabs/amazon-qldb-shell/releases/tag/v1.1.0).
* Added support for recommendation on keywords and active Table names on pressing the `Tab` key.
* `Ctrl + C` cancels the current command.

:bug: Fixes

* Error out of shell immediately when no credentials are present as reported in [issue14](https://github.com/awslabs/amazon-qldb-shell/issues/14)
* Lock driver and amazon ion versions during shell installation

:tada: Enhancements

* Relax boto version requirement on installation.

:book: Documentation

* Add unit tests.
* Correct quit to exit on README.

* Preview release of the shell : The release supports single line statements and couple of experiences may change in future with the addition of new features.
* Connect using 'qldbshell --region <region_code> --ledger <ledger_name>'.
* Interact with the ledger via the shell using a [PARTIQL](https://partiql.org/docs.html) statement per line.
* [Query Ion with PARTIQL](https://docs.aws.amazon.com/qldb/latest/developerguide/ql-reference.query.html).