Changed use of boost filesystem lib for backwards compatibility.

The current version should compile and work with v2 and v3 of Boost.Filesystem.
Notes:
 - There is a bug in replace_extension() for v3, one must include the '.' in
   the extension. Worked around by using ".FEGRID" (which includes the dot).
 - v2 is no longer available since Boost 1.48 and cannot be used.
 - v3 is available starting from 1.44, and default from 1.46.
 - We do not need to define the BOOST_FILESYSTEM_VERSION macro, since the
   syntax chosen works with both v2 and v2.
 - In the future, we may need to use v3 features, and support Boost 1.44 and
   1.45 by defining the version macro. This will drop support for 1.43 and lower.
Merge pull request #14 from atgeirr/master

Change use of Boost.Filesystem
change namespace from Dune to Opm