Add new ChainGenerator class, add .gitignore, formatting fix for README.rst

- Added a new simple graph generator - **ChainGenerator** for creating chain graphs (much like a linked-list). Created in the same way as other generators, like **FullGenerator** and **GridGenerator**.

- Added a **.gitignore** file at repo root level, same one from **gs-algo**. Standard Java and Eclipse related files and dirs ignored

- Minor formatting fix for **README.rst**