Added Python 3 support

I have added Python 3 support to libLAS. The majority of the changes are in import statements, along with one `list()` statement and a version check when creating a `File` object. For the latter two changes, see `liblas/file.py`; I will place a comment on them directly. I have also run the code in Python 2 to make sure I didn't break anything. 

The Python 2 tests failed without this change, although the failures are due to just to rounding errors. I am getting a segfault when running the doctests with Python 3. I think there needs to be another string to bytes conversion somewhere in the code.