Added possibility to define fileMode in Standard Export.
With this change it is possible to choose between write and append mode which is useful
for chunked exports.
After all rows are written flush the contents to disk
This is needed for chunked exports with append mode, because otherwise the content
still stays in buffer till php ends, which would made the chunked export useless.
Merge pull request #14 from Danez/master

Added possibility to define fileMode in standard export