BDouble String Initializer and Optionals

[twodayslate]

A better String initializer for BDouble for #19. I just used the initializer for a Double since that was already parsed using its' String representation.

>>> BDouble("no")
nil
>>> BDouble("123")
Optional(123)
>>> BDouble("1.2e10")
Optional(120000000000)
>>> BDouble("1.2")
Optional(6/5)

I removed the Fatal Error thrown when constructing an BInt and instead use optionals. I have updated the README to reflect this

I also added more descriptions

>>> var bd2 = BDouble("123453434.2341313123123131123123")!
1234534342341313123123131123123/10000000000000000000000
>>> bd2.fractionDescription
1234534342341313123123131123123/10000000000000000000000
>>> bd2.decimalDescription
123453434.2341
>>> bd2.precision = 10
>>> bd2.decimalDescription
123453434.2341313123
>>> bs2.precision = 2
>>> bd2.decimalDescription
123453434.23

Can we break up SMP.swift? I love the idea of a single file for usage but my Mac isn't handling it's huge size so well. It won't even show the diff by default on Github

FYSA, you can squash all my commits into one commit to make the repository look cleaner.

Looking forward to your thoughts. Thanks

[mkrd]

Looks good to me. I like the examples that you used here, they should also be in the readme 👍
I'll think about splitting the file up, but I prefer it like this. It's really great for people who just want to get the library without much hassle, just dragging one file into their project.