if renderCell used, api should apply the <View> wrapper with styles to the result

[pstanton]

If you use the 'renderCell' property, you cannot make the result look like a table since the column width and styles are not accessible and are not applied to the returned markup.

'renderCell' should override only the portion of the api's _renderCell function that returns the <Text/> element.

[pstanton]

	_renderCell(cellData, col) {
		let style = { width: col.width || this.props.columnWidth || DEFAULT_COLUMN_WIDTH };
		// call 'renderCell' which can return false to fall back to default implementation
		let content = this.props.renderCell
			? this.props.renderCell(cellData, col)
			: false;
		if (!content) content = <Text>{cellData}</Text>;
		return (
			<View key={col.dataIndex} style={[styles.cell, style]}>
				{content}
			</View>
		);
	}

	_renderRow(rowData, index) {
		let { columns } = this.props;
		let renderCell = this._renderCell.bind(this);
		return (
			<View key={index} style={styles.row}>
				{columns.map(col => renderCell(rowData[col.dataIndex], col))}
			</View>
		);
	}

this change also allows 'renderCell' to return false if the default behaviour is required (ie <Text/>)