Sanitize boolean conversions in signature subpackets.

[vanitasvitae]

Currently, a PrimaryUserID packet whose boolean value is encoded as an invalid, but non-zero value (e.g. 2) is interpreted as true.
I believe, BC should detect this invalid value as the sign that something went wrong and instead throw an exception.

This PR introduces the Bytes utility class in the core module, which contains two methods for converting between byte arrays and booleans.
Especially the fromByteArray() is more strict in that it does not allow arrays with a length other than 1 and also does not allow byte values other than 0 and 1.

[vanitasvitae]

The same type of sanitization can also be applied to Revocable and Exportable.
I'll add separate commits for these.

[dghgit]

Thanks for the patch. Merged!