Adding in .gitattributes file for syntax highlighting.

[rbharath]

The solidity contracts in the solidity-float-point-calculation repo currently don't have syntax highlighting. Github recently added support for solidity syntax highlighting, but due to name conflicts, requires each repo which wants such highlighting to add in a special .gitattributes file. This PR adds this file to the repo.

Contracts look very nice with highlighting turned on (for example, see https://github.com/datamined/contracts/blob/master/contracts/DataCoin.sol), so I think it will make the contracts here more readable.