Add new helper methods: BigInt, BigFloat

[mwoss]

Those are requested by the community two new helper methods that would make it easier to use the library and prevent users from making an improper decimal casting.
In the tests, I used strings for compersion as it is easier to read them in that way and it does not change result of the tests.

[njason]

Might be good to add some test cases that expect failure, like number being too big

[njason]

Or loss of precision

[mwoss]

Hm, I wasn't aware that big.Float is not available in Go v.1.2. What should we do? Drop support for that version or remove that new functionality? I'm not sure whether decimal should be still compatible with over 7 years old version fo Go.

[mwoss]

@njason big.Float is available from Go version >1.5. I would say it's ok to remove support for such old Go versions and stick to the pattern of other open-source go libraries.
If we gonna see any complaints we could revert the new BigFloat helper method.

[njason]

Sounds good to me