Bugfixes from Github Many small fixes for open github issues. See merge request !334
Closes github #51 https://github.com/espressif/esp-idf/issues/51
Closes github #128 https://github.com/espressif/esp-idf/issues/128
1. add sigma-delta code and example. 2. add gpio output signal in io matrix. Squashed commits: [a25e7d4] minor changes for doc [6a03a1e] minor changes on comments and doc. [97dd9e4] Add doc. update index.rst change example index