Fix some typos / grammatical errors
PreviousTxnID -> AccountTxnID
PreviousTxnID (deprecated) mandates the PreviousTxnID field,
 present in the transaction to match that in the AccountRoot of
the initiating Account. Note that this field is update on EVERY
transaction affecting the account (eg. 1 drop inbound payments)

See: https://wiki.ripple.com/Transaction_Metadata#Threading

Relevant source from Transactor.cpp:211

    // Deprecated: Do not use
    if (mTxn.isFieldPresent (sfPreviousTxnID) &&
            (mTxnAccount->getFieldH256 (sfPreviousTxnID) !=
    mTxn.getFieldH256 (sfPreviousTxnID)))
        return tefWRONG_PRIOR;

    if (mTxn.isFieldPresent (sfAccountTxnID) &&
            (mTxnAccount->getFieldH256 (sfAccountTxnID) !=
            mTxn.getFieldH256 (sfAccountTxnID)))
        return tefWRONG_PRIOR;
Clean trailing whitespace
Update Memos section:
Include `Internal Type` in the table
List the `MemoData` and `MemoFormat` fields (TODO: descriptions)
Add example json, as the format is "weird"
Show that the fields are variable length, and the json for this type is
a hex string
Make it clear that the 1 kbyte limit applies to the field as serialized

For further reference:
https://gist.github.com/justmoon/9462822
https://wiki.ripple.com/Example_API_Transactions

TODO:
Check the semantics and/or usage guidelines behind MemoData/MemoType/MemoFormat
Update Paths section
SendMax implies source balance when no direct trustline to Destination
Paths MUST be set when cross-currency/cross-issue payments are made
Empty Paths may not be serialized (ripple-lib doesn't serialize them,
rippled errors)

Crash script for empty Paths:

```shell

export ALICE='rG1QQv2nh2gr7RCZ1P8YYcBUKCCN633jCn'
export BOB='rPMh7Pi9ct699iZUTWaytJUoHcJ7cgyziK'
export ROOT='rHb9CJAWyB4rj91VRWn96DkukG4bwdtyTh'

function close_ledger()
{
    ./build/rippled ledger_accept
}

function submit()
{
    ./build/rippled submit $1 "$2"
}

function json-rpc()
{
    curl -X POST -v http://127.0.0.1:5005 -d "$(cat <<RPC
    {
        "method" : "$1",
        "params"  : [$2]
    }
RPC
)" | python -c 'import sys,json; print
json.dumps(json.loads(sys.stdin.read()), indent=2)'
}

submit masterpassphrase "$(cat <<TXN
    {
        "TransactionType" : "Payment",
        "Account" : "$ROOT",
        "Destination" : "$BOB",
        "Amount" : "1000000000"
    }
TXN
)"

close_ledger

submit bob "$(cat <<TXN
    {
        "TransactionType" : "TrustSet",
        "Account" : "$BOB",
        "LimitAmount" : {"value" : "500", "currency" : "USD", "issuer" :
        "$ROOT"}
    }
TXN
)"

submit masterpassphrase "$(cat <<TXN
    {
        "Account" : "$ROOT"
       ,"TransactionType" : "Payment",
        "Destination" : "$BOB"
       ,"Amount" : {"value" : "1", "currency" : "USD", "issuer" :
       "$ROOT"}
       , "Paths" : []
    }
TXN
)"

    # {
    #    "result" : {

    #       "error" : "internal",
    #       "error_code" : 59,
    #       "error_message" : "Exception occurred during transaction",

    #       "request" : {
    #          "command" : "submit",
    #          "secret" : "masterpassphrase",
    #          "tx_json" : {
    #             "Account" : "rHb9CJAWyB4rj91VRWn96DkukG4bwdtyTh",
    #             "Amount" : {
    #                "currency" : "USD",
    #                "issuer" : "rHb9CJAWyB4rj91VRWn96DkukG4bwdtyTh",
    #                "value" : "500"
    #             },
    #             "Destination" : "rPMh7Pi9ct699iZUTWaytJUoHcJ7cgyziK",
    #             "Paths" : [],
    #             "TransactionType" : "Payment"
    #          }
    #       },
    #       "status" : "error"
    #    }
    # }

```
Update SetRegularKey section
Change Internal Type to Account

An account is a 160 bit hash of a public key in compressed form (pub |
sha256 | ripemd160). The RegularKey field has the same json
representation as the Account fields, which are likewise hashes.
Merge pull request #70 from sublimator/master

merging so credit is given where credit is due; I'll update/edit as necessary