Allow for leading whitespace when finding object offsets

The PDF supplied in issue #4 contained leading whitespace on object start
tokens.  Although this is probably invalid, it's easily dealt with and is
accepted / handled by most PDF readers, so we'll deal with it too.
Use actual booleans (not strings) for boolean object values

The PDF supplied in Issue #4 contained a series of images in CCITT format,
which have a few boolean decode parameters which change the expected data.
The supplied PDF had two of these parameters set to "false", but as bools
were being stored as the strings "true" and "false", these were both being
evaluated to true by PHP, resulting in these decode params being set
incorrectly in the output, as TCPDI expects it to be a boolean.