Updated version to 0.0.6
Added support to write a character vector

* Each element in the character vector must have same length.

* Currently no support for different encodings, each element
  in the character vector is written as mat_uint8_t.

* Refactored the write_mat to use SEXPTYPE to decide how to
  write the list element.
Added test for writing a character vector.

* Added on.exit to each test that writes to file,
  to unlink file.
Updated version to 0.0.7