v0.3.0 release

Change log:

• Ignore test folder & nslist from mbed classic build
• Merge pull request #40 from ARMmbed/prepare-for-mbed-trace
• Added cmd_vprintf (#41)
• added mutex wait/release callbacks for thread safety (#42)
• add MBED_CONF_MBED_TRACE_ENABLE trace enable flag support (#43)
• Update README.md (#46)