Outdated README.md & build fails

[smartcontracts]

The current README.md directs users to build via npm build instead of npm run build. npm run build fails because of a few errors caught by the typescript compiler.

Expected Behavior

qtumjs should build correctly.

Current Behavior

qtumjs throws the following typescript compilation errors:

src/Contract.ts(230,11): error TS2564: Property '_logDecoder' has no initializer and is not definitely assigned in the constructor.
src/Contract.ts(250,31): error TS2565: Property '_useBigNumber' is used before being assigned.
src/QtumRPCRaw.ts(25,11): error TS2564: Property 'idNonce' has no initializer and is not definitely assigned in the constructor.

Possible Solution

_logDecoder

Instead of assigning contract._logDecoder as in the current codebase:

qtumjs/src/Contract.ts

Lines 246 to 248 in d36ade4

	if (opts.logDecoder) {
	this._logDecoder = opts.logDecoder
	}

_logDecoder should probably be assigned as follows:

this._logDecoder = opts.logDecoder || new ContractLogDecoder(this.info.abi)

This also removes some unnecessary logic in get logDecoder().

_useBigNumber

_useBigNumber is set via

qtumjs/src/Contract.ts

Line 250 in d36ade4

this._useBigNumber = this._useBigNumber || false

but this._useBigNumber will always be undefined so we might as well define it to

this._useBigNumber = false

idNonce

idNonce is never set in QtumRPCRaw

qtumjs/src/QtumRPCRaw.ts

Line 25 in 64b06a1

private idNonce: number

We should probably just initialize it to 0

this.idNonce = 0

Steps to Reproduce

1. npm run build

[smartcontracts]

The proposed fix passes all tests
https://github.com/kfichter/qtumjs/commit/aac5179c1827e8fa9ebafab111f1031a470aefd0