Added logging mechanism (Issue #31)

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 all log output is directed to stderr.

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().