Set default category for Skylight.instrument
Remove a stray allocation spec
Improve disabling the agent

Previously, there were two paths handling disabling the agent. One when
the agent is not started and the other when the native library is not
present. This commit unifies the two paths in favor of loading most of
the ruby code even when the native library is not present.
Increase spec robustness
Move native lib fn definitions into skylight.h

Eventually, this header file will most likely be shipped with the native
code.
Reformat C macros
Reorganize the specs
Add a spec for Batch serialization
Improve extconf.rb robustness

Resolves #97
Include .h files in gem
Add missing require in instrumenter
Bump version to v0.3.7