Add I2C periph
Update i2c.c

Minor changes (tabulations, and remove useless variables)
Remove useless variable

Remove useless variable (sercom)
Add = 0 to pass Travis test + remove @note
isr_sercom2 to isr_sercom3
Align all lines
Change comment and align define
Remove useless line (UART in I2C conf)
Replace i2c_port by i2c_pins
Some modifications (comments, line break ...)
add " missing
Minor changes

Replace sercom by sercom_core-20 and include modification + ENABLE bit in power_on to 1
Change comment style
Edit speed (3,4 Mhz)
Remove spaces
Read one more data which wasn't necessary
Add NACK before STOP!
Change makefile.features to rebase pull request
Add \n for debug
Replace tab by 4 spaces