Performance Part #1

Remove unused functions. Replace all file_exists with is_file. Fix
Exception constructor again (sorry, first time working with github).
Introduce STRLEN constants, to remove the strlen call on constant
values.
Performance Part #2

Remove unused property. Fix constant call in content node. Removed
unused code in DirectiveNode. Remove identify problem for the foreach
context variable $selector in RuleNode. Remove unreachable throw
exception call. Init $tokens in lexer. Create used SassListException.
Part #3

Remove strlen on constants (one more). Replace all is_null with the PHP
construct „ === null“. Replace magic calls (overhead with ucfirst and
method_exists) with direct getter-calls. Remove strlen call in for-loop
in SassList::_build_list.
Merge pull request #152 from lochmueller/performance_cleanups

Performance cleanups