Use byte literals in tests to make go vet happy.

The go vet command complains about untagged struct initializers when
defining a ShaHash directly.  This seems to be a limitation where go vet
does not exclude the warning for types which are a constant size byte array
like it does for normal constant size byte array definition.

This commit simply modifies the tests to use a constant definition cast to a
ShaHash to overcome the limitation of go vet.
Make ErrHashStrSize slightly more descriptive.

Add the word 'string' to the error string so if this is printed, it
will be clearer that it was a hash string decode that failed (as
opposed to other errors creating a ShaHash from a byte slice) and
change 'chars' to 'bytes', since the string length is measured in
bytes, not UTF-8 code points.

ok @davecgh
Bump default user agent to 0.2.0.
Changed TxIn.PreviousOutpoint to TxIn.PreviousOutPoint for consistency.
Fixed minTxInPayload documentation.
IMPORT PATH MODIFICATION CHANGESET
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Please use this changeset as the first one on top of the conformal codebase.
Note that if there are new imports, they need to be changed as part of this
changeset. In general, use "git rebase" on github.com/conformal's version
and apply this changeset first.