EIP 1193 Improvements

[ryanio]

This PR targets some small improvements for EIP 1193:

1. Allows params to also be type Object to support by-name variables (supported in both JSON-RPC and GraphQL specs).
2. Adds language to encourage support of multiple transport methods beyond JSON-RPC.
3. Improves introduction.
4. Removes own error table in favor of error codes listed in EIP 1474.
5. Removes sample class implementation.
   1. I'm not sure if it's useful to include, it was just my reference implementation while working on Mist.
6. Some sentence and grammar improvements to improve clarity.

I posted this PR on my own fork because otherwise the the EIPs GitHub repo would auto-merge it before we could have any discussion.

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The EIP has a few remaining discussion items before I would like to propose to move to "Last Call":

1. Renaming send to request
   1. See Discussion: EIP-1193 ethereum/EIPs#2319 (comment)
2. Provider capability introspection
   1. See Discussion: EIP-1193 ethereum/EIPs#2319 (comment)

[ryanio]

Some of these changes have been applied in ethereum#2577