Add hardware serial support

[NickNothom]

It was a little confusing to get this library working for hardware serial, now you can just use the compiler flag.

If this looks good, I'll update the docs to match.

[patrickelectric]

Hi Nick! Thanks for the PR.
Some points:

• The first commit is a bit generic, it will help to add at least a (simple example) in the end.
• The commits e570a23 and 7207b03 should be squashed since they are changing the same file for the same thing, there is no big reason to have two commits.
• Comments is a really generic commit title without body, and it appears that you are fixing what you did in d59069f, this should be squashed to avoid unnecessary commits.
• Also, it appears that your code is failing to build, you can check that in travis:

ping1d-advanced:90:29: error: 'class Ping1D' has no member named 'gain_index'
         Serial.println(ping.gain_index());