Fixed a bug that crashed python core on null values

This is analogous to line 517-519.
pyshark_getValueWithType may return NULL which is not an acceptable value for
PyDict_SetItem (doc?).
More useful results for unnkown fields

Added a small "hack" to return the hex-dump of the field value, when
libwiereshark doesn't know what to do with it. The original behaviour was to
simply return the name of the field as its value.
Added support for homebrew Mac OSX systems

Tested with wireshark 1.8.6 and works.
Merge pull request #4 from ohm314/master

Fix for pyshark.c, more useful info printed by sharktools_core, and configure script support for homebrew MacOSX systems.  Thanks, Omar Awile!