Commit to enable export/import of bit array for persistance.

Hello, I liked the simplicity of your bitt array and want to use it my code. However, I need to persist the bit array and needed and import/export ability. 

Additionally, on the constructor, I added a check for the length. If the length is 0 or negative, you have problems. 

I hope you find this useful.

Sincerely,

Edmond