Add validation of invariants for PublicIdType and ImmutableData, see …

[ghost]

…comment at MAID-1056. Fixes #57.

[maidsafe-highfive]

r? @dirvine

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[dirvine]

Review status: all files reviewed, all discussions resolved, some commit checks pending.
Reviewed files:

• src/data/immutable_data.rs @ r1
• src/helper.rs @ r1
• src/id/public_id_type.rs @ r1

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Comments from the review on Reviewable.io

[coveralls]

Coverage increased (+0.3%) to 92.74% when pulling 4140731 on s-brian:MAID-1056 into 4049d56 on maidsafe:master.