Fix 'requestFrom' rollback issue and add a little more bullet proofing

[ghost]

• Added 'busy' flag to enable slave time to finish processing previous request.

• Moved the setting of SDA as input to SET_USI_TO_TWI_START_CONDITION_MODE. This helps to prevent conditions where SDA is held low indefinitely after a SET_USI_TO_SEND_ACK or SET_USI_TO_SEND_DATA, due to a missed clock pulse.

• Added macro SET_USI_TO_TWI_OVERFLOW_MODE.

• Added a timeout to wait for proper start condition latch. Helps with second point above.

• Added function usiTwiHandleSTOP which is called when a stop condition is detected via TinyWireS_stop_check. This ensures use of new 'busy' flag.

• TinyWireS_stop_check modified to use usiTwiHandleSTOP.

• Reinstated multibyte 'requestFrom' by master functionality.

• Added tws_delay_micros function.

• Added a few more comments and reduced some confusion resulting from mixing up the roles of 'Master read' and 'Master write' with what the slave is doing. I think that one actually came from the app note and original source!

• Tested with ATtiny85 (1MHz) and SAMD21 (48MHz) as master.

• Fixes issue #44 . Potentially helps with issues #24 , #36 , #43 and #47 ?

[rambo]

hmmh ? merged to somewhere else ?

[ghost]

I'd done some more work on this recently and modified things a little outside the scope of the original pull request. I merged this work from 'testing' into my master.

From testing, the mods I've made has solved a lot of my issues but I'm not sure if it is of any use to anyone else. Feel free to grab whatever you may find helpful for your own projects.