Fix issue #161

Moved the logic for creating a buffer to allow direct access to the
characters in a string into the StringUtil class. The code was fixed to
always create a buffer that is large enough.

Changed AbstractFormatter and FormetaParser to use the new method in
StringUtil.
Updated PicaDecoder to use StringUtil.copyToBuffer.

The original implementation in PicaDecoder had a bug (see issue #161).
By refactoring the code to use the new StringUtil.copyToBuffer method
this is fixed.
Merge branch 'fix-issue-161' of https://github.com/cboehme/metafacture-core into cboehme-fix-issue-161