Standardize some whitespace

The preferred style is 4 spaces.
Refactor textsecure.protos -> textsecure.protobuf

DRY up protobuf declarations and move to a slightly briefer naming
convention.

Also dropped some ArrayBuffer -> string conversions as
ProtoBuf.js handles ArrayBuffers just fine, and in fact, more
efficiently than strings.

Finally, dropped the btoa() wrappers, because that incurs an extra
string -> string conversion before the protobuf's internal string ->
array buffer conversion. In lieu of btoa, we can simply pass in the
optional string encoding argument to the protobuf's decode method,
which in these cases should be 'binary'.

Related: #17