Fix codestream_info memory leak

[lpartington]

1. The return value from opj_get_cstr_info has to be freed by calling opj_destroy_cstr_info.
2. It is convenient to allocate the output image buffer (decoded_) before calling opj_decode; this was observed to allow failure sooner if there wasn't enough memory to decode AND return the image.
3. Minor correction to readme - it appears that openjpeg master branch is used now
4. decoded_ buffer is cleared at start of decode_i() to reduce memory footprint (i.e. before openjpeg objects are allocated)