Beta of Major Refactors

Minimum rustc version bumped to 1.22 (#254)

Added:

• Nil variant to UuidVersion (#181)
• Add UuidVersion to prelude (#188)
• support for u128 (#201)
• Allow v3 & v5 to be constructed from bytes (#270)
• BytesError for general Uuid use (#284)

Changes:

• Move namespaces into Uuid::(#187)
• Use UuidBytes instead of [u8;16] (#263)
• from_bytes renamed to from_slice, from_uuid_bytes renamed to from_bytes (#299)
• Uuid Prefix removed from functions (#294)

Removed:

• Uuid::new() method (#214)
• Remove deprecated use-std feature (#280)

Dependencies:

• Rand to 0.5 (#262)

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