Bug fix of potentially dangerous "head" call.

Code to ensure the call to head is done safely when the stack is
empty.
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