Improvements to OverloadedMethod

[iamdefinitelyahuman]

What I did

Allow calls to OverloadedMethod without specifying the function signature. Closes #348

How I did it

When an overloaded method only has one function with a given number of arguments, do not require the signature to be explicitly declared.

How to verify it

Run tests. I added test cases.

[codecov-io]

Codecov Report

Merging #507 into master will increase coverage by 0.02%.
The diff coverage is 92.68%.

@@            Coverage Diff             @@
##           master     #507      +/-   ##
==========================================
+ Coverage   88.46%   88.49%   +0.02%     
==========================================
  Files          56       56              
  Lines        6145     6178      +33     
  Branches     1357     1366       +9     
==========================================
+ Hits         5436     5467      +31     
- Misses        485      486       +1     
- Partials      224      225       +1     

Impacted Files	Coverage Δ
brownie/network/contract.py	90.53% <92.50%> (-0.17%)	⬇️
brownie/network/transaction.py	87.52% <100.00%> (ø)
brownie/convert/normalize.py	86.07% <0.00%> (+2.53%)	⬆️

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