Add test for binary STLs with 'solid' header and fix STLDocument

[MHebes]

Fixes #26

@JoshMcCullough ping

[JoshMcCullough]

Thank you, looks good. Couple tweaks...

[MHebes]

@JoshMcCullough Responding to all your comments at once: It appears that const in C# is only allowed for numbers, Boolean values, strings, or a null reference and is not equivalent to the const keyword in e.g. JavaScript.

It is my understanding that const in C# means "has static storage and can be initialized at compile time" rather than "cannot be changed post initialization". The latter refers to readonly, which is only applicable to class members and not local variables.

[MHebes]

Happy to adjust whitespace or formatting as you wish, though. I had a lot of trouble getting the pre-push git hook working on windows + git bash.