Expose feature activation fields when decoding transaction

[dexX7]

Expose feature activation fields when decoding transaction to support parsing of feature activation transactions.

This resolves #314.

[zathras-crypto]

Reviewed & tested, all looks fine:

$ src/omnicore-cli --testnet omni_gettransaction 21c208f6b01274e509f32ccebf74b9c886d1c78c664b7f9121ccc8f805d23c22
{
    "txid" : "21c208f6b01274e509f32ccebf74b9c886d1c78c664b7f9121ccc8f805d23c22",
    "fee" : "0.00003804",
    "sendingaddress" : "mpZATHupfCLqet5N1YL48ByCM1ZBfddbGJ",
    "ismine" : true,
    "version" : 65535,
    "type_int" : 65534,
    "type" : "Feature Activation",
    "featureid" : 1,
    "activationblock" : 530285,
    "minimumversion" : 999,
    "valid" : false,
    "blockhash" : "000000000000060725730562ffdc22e2ba38babbc1bc8336afafc4df1070e345",
    "blocktime" : 1439455392,
    "block" : 530276,
    "confirmations" : 99076
}

OK to merge :)