Add preTransmission(), postTransmission() for half-duplex

[kintel]

This is a suggested API change to address dealing with RS-485 transceivers in half duplex mode (Issue #33).

I'm not married to this particular way of solving it, but it's tested to work on my end.

[4-20ma]

Interested in the PR. Please fix conflicts and resubmit. Thanks.

[kintel]

Merged with master and resolved conflicts.

[4-20ma]

Spelling: "transcieiver's"

- ...RS485 transcieiver's
+ ...RS485 transceiver's

[4-20ma]

Thanks for the PR, though I think after I incorporated #43, it introduced new conflicts. I've marked up the PR with minor edits (spelling, capitalization, spacing, & convention). If you'll incorporate these and fix the merge conflict, I'm ready to merge.

[4-20ma]

@kintel it occurred to me that we should include a simple example showing off the half-duplex feature. Do you have a simple bare-bones example to include?

[kintel]

@4-20ma I agree that an example would be nice. I can clean up one of my test sketches and submit that as a separate PR.

Thanks for being thorough btw. - it really helps knowing that someone actually read the code :)

[4-20ma]

LGTM

[4-20ma]

I look forward to a simplified example that shows off how to use the half-duplex feature. Thanks for implementing. Travis CI is now implemented (compiles each example included in examples/) so it will catch the obvious build errors automatically.