Improve test coverage

[abingham]

We could use better test coverage. I'd like to use this issue a place to keep track of what we need to do. At the very least I'd like a list of areas that could use some work. We could also use this as a meta-issue and link it to separate issues for specific testing efforts.

[rob-smallshire]

main.py	not covered
cli.py	not covered
dataset.py	not covered
reader.py	not covered
writer.py	not covered
encoding.py	31% lines covered
toolkit.py	49% lines covered
revisions.py	53% lines covered
util.py	56% lines covered
header.py	64% lines covered
docstring.py	65% lines covered
packer.py	70% lines covered
ibm_float.py	76% lines covered
catalog.py	82% lines covered
field_types.py	90% lines covered
datatypes.py	92% lines covered
init.py	100% lines covered
binary_reel_header.py	100% lines covered
sorted_set.py	100% lines covered
textual_reel_header.py	100% lines covered
trace_header.py	100% lines covered

[rob-smallshire]

As part of this let's get Coveralls up and running. I prefer Coveralls to that thing we use for Cosmic Ray just because it's less intrusive and chatty.