point compression bug fix

There was a problem with point_uncompress. nx or ny may be too short if they would otherwise start with '\x00'. The null byte(s) must be re-added. 

Also I couldn't figure out why you were calculating nx instead of just using x except perhaps to keep it clean looking so I took it out for performance reasons. I obviously also changed point_compress back to use a tuple so that it is compatible with point_uncompress.

I've run several hundred thousand tests with randomly generated secp256k1 keys without any problems. How do you feel about your code? Are you confident in it? Why did Yann not merge it? I ran the test at the bottom of the other pull-request discussion and it passed (https://github.com/yann2192/pyelliptic/pull/9). 
Please let me know.
Thank You,
Atheros