Fix/contract call-#218

[j-chimienti]

fixes #218

issue:

finding contract abi via name runs into errors if multiple functions w/ same name

changes:

updates to ethFuncs, uiFuncs, and contract class to handle multiple functions in abi w/ same name

1. contract class and ethFuncs / uiFuncs accept contract interaction to accept function or name of function

   • see ethFuncs.prepContractData for ex

2. Promisify sending tx or executing contract to make code more maintainable

   • separate error handling from casual code
   • remove nesting and awkward code patterns

3. reduce rewritten code for user notification

   • sending tx / contract function displays errors to user (successful txs default as well)

testing

https://github.com/EthereumCommonwealth/etherwallet/issues/218#issuecomment-415083751

[j-chimienti]

fixes #241

Validation

1. clear rawTx and signedTx if user changes values

2. add tx unit

3. set gasLimit and value as numbers

4. gasLimit min = 21,000

5. value min = 0

todo:

1. add tx unit select
2. set max value of tx.gasLimit based on api call