add support for encoding and file extension for csv export

this pr solver the issue https://github.com/camptocamp/cgxp/issues/521

it implement the possibility to specify the encoding and the file extension for the csv export

utf8 and .csv have been tested ok with the following programs:
Excel 2007
Excel 2010
LibreOffice 4.0.4.2
LibreOffice 3.5.7.2

not ok with:
Excel 2002 (no utf8 support at all)
OpenOffice 3.3.0 (utf8 not recognized automatically, but can chose the encoding on start)

linked to https://github.com/camptocamp/cgxp/pull/556