Add File::availableForWrite API

[sandeepmistry]

This can allow sketches to see if the write will be non-blocking.

[TravisBuddy]

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[sandeepmistry]

@cmaglie @facchinm this part is a bit dangerous, since we no longer wait for the result of the write command.

In the SD card I used for testing, the waitNotBusy would spin for 10 - 20ms.

[facchinm]

We should take extra care and understand how people really use this library (like continuous blocking writes or other patterns).
Said that, I love this patch!

[sandeepmistry]

I think we can only activate this non-blocking behaviour if someone calls availableForWrite(). This will allow older sketches to continue with the existing code path.

Stay tuned, I'll push some more changes later today.

[per1234]

Regarding the Travis CI issue, I have verified that it is fixed by #67. That could be merged now if you want to get the CI tests working again.

[TravisBuddy]

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[TravisBuddy]

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[TravisBuddy]

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[TravisBuddy]

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[TravisBuddy]

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[sandeepmistry]

This is ready for review now :)