Signature decoding tests

Added tests of Signature decoding to illustrate a problem decoding
a signature that exists in the block chain.  The signature has strange
DER lengths.
Bug fix:  Incorrectly reporting the DER length in
the error message
Removed code assertions on DER size when parsing signatures.

This is required to sucessfully verify block 124276, transaction index 3
( fb0a1d8d34fa5537e461ac384bac761125e1bfa7fec286fa72511240fa66864d )
input 0.
Merge pull request #34 from lornap/sigDecodeFix

Fixing Signature Decoding Problem