Add unit tests

[pjreed]

Given how delicate reading in bag files is, it'd be nice to have some unit tests. A few things that should probably be tested:

• Reading in all of a bag's headers
• Uncompressing a compressed data chunk
• Deserializing each of the primitive types
• Deserializing a complex message type

[pjreed]

Some work was done on this in 1.9.0. The current set of unit tests now read in a set of sample bags that test deserializing both scalars and arrays of all of the primitive types. It'd be good to add tests for compressed data, too.

[pjreed]

I added in a PointCloud2 bag in #29 that is LZ4-compressed and also demonstrates how to read in arrays of complex message types.