Added variable-level logging mechanism

Added a mechanism to log messages with different priority levels, and to
filter which messages are output by setting a threshold level. The
function to set the current log level/verbosity is exposed through
libbladeRF.h, while the logging function/macros themselves are private
to this project, and are present in log.c/log.h. Callers should use the
convenience macros bladerf_log_[level]() to issue their log messages.
These macros piece together a format string that contains the message
priority and, for warning and higher priorities, the function and line
number on which the message was logged. Also, warning level and higher
level messages are output to stderr, rather than stdout.
Replaced dbg_printf's with new logging mechanism

Replaced all of the dbg_printf calls in the library code with logging
calls that utilize the new logging mechanism. I just made some educated
guesses at the most appropriate log level for each message.
Merge pull request #74 from frank8371/issue31_squashed

Added logging mechanism (Issue #31)