opentxs-api: use nullptr instead of NULL

Command used:
    sed -r -i 's/([^[:alpha:]])NULL([^[:alpha:]]|$)/\1nullptr\2/g' "$file"
opentxs-core: use nullptr instead of NULL

Command used:
    sed -r -i 's/([^[:alpha:]])NULL([^[:alpha:]]|$)/\1nullptr\2/g' "$file"
opentxs-ext: use nullptr instead of NULL

Command used:
    sed -r -i 's/([^[:alpha:]])NULL([^[:alpha:]]|$)/\1nullptr\2/g' "$file"
opentxs: use nullptr instead of NULL

Command used:
    sed -r -i 's/([^[:alpha:]])NULL([^[:alpha:]]|$)/\1nullptr\2/g' "$file"
opentxs-script: use nullptr instead of NULL

Command used:
    sed -r -i 's/([^[:alpha:]])NULL([^[:alpha:]]|$)/\1nullptr\2/g' "$file"
opentxs-api include: use nullptr instead of NULL

Command used:
    sed -r -i 's/([^[:alpha:]])NULL([^[:alpha:]]|$)/\1nullptr\2/g' "$file"
opentxs-core include: use nullptr instead of NULL

Command used:
    sed -r -i 's/([^[:alpha:]])NULL([^[:alpha:]]|$)/\1nullptr\2/g' "$file"
Merge pull request #133 from toxeus/useNullptr

Good job :1: