Stateless password hash derivation with header specificity

[michaeljgray-sfdc]

This pull request closes #12 by exposing a static method to perform header based and stateless encoding for passwords. I exposed the SaltLength field as a public static readonly field instead of private const to ensure that callers know the buffer size in a deterministic way; the reason for readonly over const is to prevent recompilation of large dependencies if/when the value changes in the future. This pull request also adds an optimization to reduce frequent re-allocation of the salt buffer for each encoding operation performed by instance methods.

This also adds the .vs directory to .gitignore to prevent Visual Studio client settings from being included.