Added logging mechanism

Added a variable-level logging mechanism and replaced dbg_printf() calls in the library code with calls to the new debug functions, selecting appropriate log levels for each message to the best of my ability.

Note that messages logged at BLADERF_LOG_LEVEL_WARNING and higher have a few differences from other messages:

1.) They contain function and line number information prepended to the output string
2.) They are printed to stderr rather than stdout
3.) They are fflush()'d after output (this probably isn't necessary since they're going to stderr, anyway, but it's not harmful, either)

Note also that the the default threshold for displaying log messages is BLADERF_LOG_LEVEL_INFO. Messages written at lower log levels will not be displayed unless the log threshold is changed with a call to bladerf_log_set_verbosity().