Fix for android C++ stdlib not having std::to_string.
Use convention to reduce repetition.
Rename friend-not-found error codes.

All error codes that mean "friend number didn't designate an existing
friend" are now called FRIEND_NOT_FOUND.
Renamed some functions for more consistency.
Rethrow protobuf exceptions.

Exceptions in this place would mean something very bad happened, and we
want to know about that.
"nospam" is actually a word in the Tox world.
nullptr is a valid input (causes error) for some functions
Protobuf source code reformatting.
Removed .keep file now that there are actual tests.
Consistency in comments
Added a bunch of error path tests.
Add very simple beginning of client library + spec.

And a dummy Main that does nothing for now.
Change specification of effect of sending less than requested.
Merge branch 'sk8ter-master'
Remove broken tests
Merge branch 'pippijn-master'

Conflicts:
	src/test/java/im/tox/tox4j/ToxCoreTest.java
Fix sign-compare warning for tox_file_data_size