Provide convenience method to get Method by name from abi.{Contract|Interface}

[barnjamin]

Problem

I expect the most common use of the Contract/Interface objects will be to read some json and use it to populate ATC method calls.

We should provide an easy way to access the methods by name, they're currently stored in a list

Solution

Implement a method get_method_by_name that accepts a name string and searches for a matching method, returning it if only 1 is found.

If none or >1 are found, raise error

PRs

Tests: #185 (MERGED)
Python: algorand/py-algorand-sdk#345 (MERGED)
JS: algorand/js-algorand-sdk#583 (MERGED)
Go: algorand/go-algorand-sdk#330 (MERGED)
Java: algorand/java-algorand-sdk#337 (MERGED)