Add hash_SHA_1, hash_SHA_256, hash_SHA_512 extension methods

[DinisCruz]

public static string hash_SHA1(this string target)
public static string hash_SHA256(this string target)
public static string hash_SHA512(this string target)

related to #1

https://msdn.microsoft.com/en-us/library/system.security.cryptography.sha1(v=vs.110).aspx
https://msdn.microsoft.com/en-us/library/system.security.cryptography.sha256(v=vs.110).aspx
https://msdn.microsoft.com/en-us/library/system.security.cryptography.sha512(v=vs.110).aspx