bci: Add helper bci_ad_get()

This patch adds a helper function to extract advertising data types from a
given buffer. This function returns true if the buffer contains valid encoded
data.

Usage example:
result = bci_ad_get(buffer, sizeof(buffer),
		BCI_AD_NAME_COMPLETE, name,
		BCI_AD_TX_POWER, &tx,
		BCI_AD_INVALID);
examples/bci-broadcaster: Use bci_ad_put() to assemble data
nrf51822: Add README
examples: Update README
Update README