Tinkerings.

1. Correctly expose constants as documented.
2. Adding "delay" as a property in the C wrapper.
3. Changed internal function full_duplex_transfer to actually accept
speed, delay and size as arguments.
4. Passing aforementioned values into spi_ioc_transfer, which is
ignored currently.
Document fix on options.
formatting
formatting / uncommented device open / prep rx buffer so it is the same size as the tx buffer
minor re-wording
getters and setters for options / more examples in README

added:
  mode()
  chipSelect()
  bitsPerWord()
  bitOrder()
    ORDER_MSB
    ORDER_LSB
  maxSpeed()
  halfDuplex()
  loopback()

Not all options work on all platforms.
wording fixes
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return this._spi to enable option chaining
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Merge pull request #2 from anders94/master

node-spi: added getters / setters and some examples