* tiff defines DataType and constants for the datatype values

Changes to allow this code to work more easily in other build environments
* exif_test ignores non-jpg files in the samples directory
* add test_sample_dir flag to tiff_test and exif_test to tell them where to locate the test inputs
Update covnertVals and TypeCategory to use new constants for DataType
Switch to test_data_dir instead and include exif_test::TestMarshal in the use of test_data_dir
doc tweaks.
created type+constants for raw tiff tag data types. parametrized sample test file dirs. minor tweaks.
go fmt
Merge remote-tracking branch 'upstream/go1' into go1