Added DTM wrapper, supports training on a corpus


What is included
-----------
* Initial implementation for the DTM wrapper. Made it look  like the Malletwrapper as much as possible (for consistency)
* Included a test_dtm script  to test out the model. I thought of putting this in the unittest, but was not sure. 


To Run the test_script

* You just need to set the DTM_HOME variable or just supply the path to the binary.
* You can get the binaries either by compiling them from  here or here (also has precompiled ones for Linux and Windows) 
* Supply another corpus if you wish .



The wrapper is currently missing inference on a holdout corpus.


 [1]:https://code.google.com/p/princeton-statistical-learning/downloads/detail?name=dtm_release.tgz&can=2&q=
 [2]:  https://github.com/magsilva/dtm