return errors instead of panicing
String output, err, and doc updates. Other tweaks.

* renamed TypeCategory to Format and made format calculated upon decoding
  rather than repeatedly for every format call

* unexport BadTypecastErr -> wrongFmtErr

* Change String method to just return the string value - and don't have
  square brackets if only a single value

* add separate Int and Int64 retrieval methods

* update docs
fix mknote StringVal call
Merge branch 'nopanic' into camlistoreBackport

Required some changes wrt. to new API in exif.go