[Issue #11] ENVI 8-bit data type is unsigned.

- Correctly maps ENVI data type "1" to 8-bit signed integer
- Raises an exception when attempting to save an image data type not
  supported by the ENVI format (e.g., 8-bit signed integer).
- Adds function get_supported_dtypes to return list of supported
  image data type names.
Merge branch 'issue-11' [closes #11]