Fix InflateDimensions to not throw an error

* Removed throwing an error from InflateDimensions when the data type
  not equals MAT_T_INT32. The checking has to be performed by the caller.
Added matlab variable class MAT_C_OPAQUE

* src/matio/matio.h: Added MAT_C_OPAQUE = 17 to enum matio_classes
Added OneOnly.mat test case data

* inst/extdata/OneOnly.mat: Added Matlab generated file that causes the
  following problem when reading:

  Error in read.mat("OneOnly.mat") :
  InflateDimensions: Reading dimensions expected type MAT_T_INT32
Add read OneOnly.mat test case

* tests/one_only.R: Added file with test case to read OneOnly.mat
Add .travis.yml
Fix read structure arrays recursively

* src/rmatio.c (read_structure_array_with_fields): Fix to read a
  MAT_C_STRUCT when reading a structure array.
Fix to read MAT_C_EMPTY and MAT_C_OPAQUE in structure

* src/rmatio.c (read_structure_array_with_fields): Fix to read
  MAT_C_EMPTY and MAT_C_OPAQUE in structure array and not signal
  error.
Removed raising an error when reading MAT_C_FUNCTION

* src/rmatio.c (read_mat): Instead of raising an error when reading
  MAT_C_FUNCTION a NULL value is added to the list
Merge pull request #2 from stewid/work-on-issue-1

Work on reading function handles