Removed unnecessary 'extern C'-block

Since handleInterrupt always sets to changeCount a bit later, this does not
change program behaviour. It does, however, make it easier to understand the
code, and in particular, to verify that no overflow occurs here. See issue #44.

This commit mainly adds comments, and gets rid of one else-if, so it
should be easy to see that no functionality was changed so far.

It is now much simpler, results in a smaller binary, and is more
similar to the other getCodeWord methods

Also moved it to be before getCodeWordB, so the getCodeWord* methods
are now sorted alphabetically.

Overall, the RCSwitch code is a mess when it comes to uniform
formatting; but overall, it seems using 2 space indention is
predominant, so let's try to make sure new code uses that, and slowly
migrate existing code to use it.

Some compilers rightfully complain about this; it could also fool
somebody into thinking a string was returned, when in reality
that's not the case.

The new code is smaller and faster and perhaps also easier to understand

fix 2 bugs introduced in PR #35

Test case:
https://gist.github.com/vgheo/f34ac7afd6447b3f1fc4

Iterate over all defined protocols in `handleInterrupt`

Travis integration + Readme changes

Fixed a dumb error (duplicate lines)

So far, while decoding the timings, the `handleInterrupt`routine only
iterated over protocols 1-3 before failing.

This small change will iterate over *all* protocols that have been
defined.

Tweak initial high signal of protocol 3

This resolves issue #32 (with a recent Globaltronics GT-FSI-07), and also is necessary
for compatibility with my Brennenstuhl RCS 2044.

Adds compilation support for Raspberry Pi

Resolves #8