Fix PHP_CodeSniffer errors

Fix the following errors reported by PHP_CodeSniffer:

FILE: .../tweeper.php
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FOUND 1 ERROR AFFECTING 1 LINE
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 1 | ERROR | [x] The PHP open tag must be followed by exactly one blank line
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PHPCBF CAN FIX THE 1 MARKED SNIFF VIOLATIONS AUTOMATICALLY
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FILE: .../src/Tweeper.php
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FOUND 6 ERRORS AFFECTING 2 LINES
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 373 | ERROR | [x] Incorrect spacing between argument "$host" and equals sign; expected 1 but found 0
 373 | ERROR | [x] Incorrect spacing between default value and equals sign for argument "$host"; expected 1 but found 0
 373 | ERROR | [x] Incorrect spacing between argument "$validate_scheme" and equals sign; expected 1 but found 0
 373 | ERROR | [x] Incorrect spacing between default value and equals sign for argument "$validate_scheme"; expected 1 but found 0
 388 | ERROR | [x] Inline comments must start with a capital letter
 388 | ERROR | [x] Inline comments must end in full-stops, exclamation marks, colons, question marks, or closing parentheses
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PHPCBF CAN FIX THE 6 MARKED SNIFF VIOLATIONS AUTOMATICALLY
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FILE: .../autoload.php
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FOUND 1 ERROR AFFECTING 1 LINE
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 1 | ERROR | [x] The PHP open tag must be followed by exactly one blank line
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PHPCBF CAN FIX THE 1 MARKED SNIFF VIOLATIONS AUTOMATICALLY
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Time: 260ms; Memory: 10Mb