1. Rename api from rmt_get_ringbuf_handler to rmt_get_ringbuf_handle (Github Issue #274)
2. doxygen param rename
3. change API rmt_wait_tx_done to specify maximum waiting time
4.Change return type of rmt_wait_tx_done API from bool to esp_err_t
5. update doc
* Use "example" in all example function & variable names,
ie use i2c_example_xxx instead of i2c_xxx for example functions.
Closes#198https://github.com/espressif/esp-idf/issues/198
* Mark example functions, etc. static
* Replace uses of "test" & "demo" with "example"
* Split the UART example into two
* Rename "main" example files to end with "_main.c" for disambiguation