Support for BrainVoyager VTC files

After talking to Emanuele Olivetti and Yaroslav Halchenko about it I submit this pull request for implementing support for BrainVoyager QX files. I started out with VTC files and read and write does work. As this is my first major pull request I would really appreciate some comments or help. I did not write nose tests yet and I will continue to implement support for .msk (mask) files (this should be straight forward) and .vmp files (a little more difficult).

I hope that my implementation is not too "dirty" as it was a little difficult for me to implement the variable-length header of BrainVoyager files...