Passing down XL flag into each box parser.  #212

This should prevent having to call the tell method on the file pointer.
Refactored dtbl box to use just a single file read.  #212
Refactored PCLR box parsing to use a single file read operation.  #212
Refactored reader requireents box to use just a single file read.  #212
Removed box_is_XL parameter.  #212

We only get around a 1% performance increase, which is not worth the
increased complexitiy.
main_header of Codestream object is now a property.  #212

This improves loading time of glymur.data.jpxfile() by 33%.
Added set and get_parseoptions to control codestream parsing. #212
Merge branch 'issue200' into issue212
Fixed test on python2.7
Merge branch 'issue212' into devel