Add support for multiline pica records.

The pica parser accepts new lines (\x0a) in the input as a field
delimiter. This makes it possible to parse not only pica plain but also
multi line pica records.

The test cases were refactored to make the input being tested more
visible.
Handles record markers in pica records gracefully (fixes #139).

Modified the pica parser to behave as if a field marker was encountered
when a record marker is encountered. Strictly speaking this is not
correct behaviour but the PicaDecoder expects single records in the
first place. Consequently a record marker should only be found at the
beginning or the end of the input.
Improved documentation of PicaDecoder.

Added an in-depth description of the parser behaviour.
Merge pull request #141 from cboehme/multiline-pica-parser

Multiline pica parser