ENH: Faster reading of sigmet files and support for missing rays

* Sigmet files are read as speeds similar to RSL using the Cython SigmetFile
  object.
* Sigmet files which contain rays which were not collected are returned as
  proper radar object with the missing rays excluded from the data.
ENH: Two addition to sigmet files reading.

* read_sigmet can now accept a file-like object as the filename parameter.
* 1-byte moments can are now converted by SigmetFile objects.
BUG: fixed reading of sigmet files with ray data to the end of the record
ENH: reading of sigmet files with extended headers
Merge pull request #85 from jjhelmus/master

ENH: Faster reading of sigmet files and support for missing rays