Fix parsing device descriptions with character entities.

The characters() SAX handler may be called multiple times for the
same XML element when there are XML character entities. We need to
append the characters to the current string value.
Merge pull request #61 from jlai/sdk_1.2-dev

Fix parsing device descriptions with character entities.
Merge branch 'sdk_1.2-dev' into sdk_1.3-dev
Wifi toggle will clear discovered devices, fixes #63