Initial commit for OpenFOAM

# What is OpenFOAM?
OpenFOAM (www.openfoam.com and http://www.openfoam.org/docs/user/) is a C++ library to simulate physical flows numerically and very popular in academia as well as in the industry. It runs under linux, is command line based and each simulation case consists of a lot of files of which some are worthwhile to track and others don't. This gitignore file ignores the second category.

Unfortunately there is no documentation or specification available that states which files are generated. I hence compiled this list based on experience and validated it on some example cases.

# What is included in this pull request?
* Ignore any time-step folder
* Ignore any processor directory
* Ignore mesh related data that is not a dictionary
* Ignore flux fields
* Ignore logfiles
* Ignore paraview/paraFoam related files

# Commit message
Tested with following tutorials out of $FOAM_TUTORIALS:

1. mesh/snappyHexMesh/motorBike
2. incompressible/simpleFoam/motorBike
3. basic/potentialFoam/pitzDaily