export possibility to parse an arbitrary std::istream

this has the potential to reduce the memory requirements of the parser
for opm-benchmarks considerably and is quite easy to add since
internally all parsing happend on istreams anyway...

The parser integration test has been extended to test the new method
as well as `parseString()` which was omitted previously. (I wonder who
introduced this without changing the test. ;)
Merge pull request #216 from andlaus/add_parseStream

export possibility to parse an arbitrary std::istream