Bug fix of potentially dangerous "head" call.

Code to ensure the call to head is done safely when the stack is
empty.
Removing block batch option in the SPVNode and default to 500 blocks
Bug fix: Failure handling in P2SH script decoding

Preventing a failure to decode from crashing the application.
Merge pull request #122 from plaprade/master

Removing block batch option in the SPVNode and default to 500 blocks
Merge pull request #123 from lornap/fixDangerousHead

Bug fix of potentially dangerous "head" call.
Merge pull request #124 from lornap/fixDangerousDecode

Bug fix: Failure handling in P2SH script decoding