added test for signing P2SH
fixed a mistake made in 4ef7ddc5e867ad8ca6562851088c374884e5d427 that inverted the length check.
this resulted it RawTransactions sometimes thinking it's own signatures were not valid.

fixes #28.
Merge branch 'bip32-absolute-path' into blocktrail
Merge branch 'canonical-signature' into blocktrail