Remove .gcda files to prevent "invalid arc tag" warnings.
Remove bogus SPDYProtocol assert.

We do, in fact, write 0 bytes. Sometimes the final data frame is
simply a marker for "end of stream" but has no body, and the SPDYSession
code doesn't check for this case.
Minor public API changes for logging and metadata.

Add currentLogger method.
Add currentLoggerLevel method.
Return nil instead of an empty dictionary when metadata not available.
Expose SPDYError.h as a public header.

Split out the helper macros into SPDYDefinitions.h, and clean up
internal usage of the headers.

Also moves SPDYError.h to the "public" headers in Xcode.
Merge pull request #88 from kgoodier/develop-next

Minor public api and cleanup changes.