Add VariableDeclarationTuple rule

[cgewecke]

This PR

• adds a rule to allow tuple formatted variable declarations like var(x,y) = (10, 20);.
• adds examples of tuple use to doc_examples.sol
• restores the VariableDeclarationNoInit rule that was removed in #36. It looks like using VariableDeclaration to implement the tuple rule would allow var(x = 5, y) = (10, 20)?
• Resolves #9. (A sample AST for this proposal can be found there).

NB: The code in this PR was authored by @duaraghav8 as part of solparse. He has given permission to integrate his bug-fixes into this repo so that solidity parser efforts can be consolidated in one place.

[federicobond]

@cgewecke keep in mind that this is also valid code:

var (,a) = fn();
var (b,,) = fn();